NRLF 


B   M   175   77i4 


IBRARY 

DIVERSITY  OF 
CALIFORNIA 


ARTK 

cie 


METHODS    IN 
PRACTICAL    PETROLOGY 


LONDON    AGENTS: 
SIMPKIN,    MARSHALL    &   Co.    LTD. 


Methods  in 
Practical   Petrology 

HINTS   ON   THE    PREPARATION    AND 
EXAMINATION    OF    ROCK    SLICES 

BY 

HENRY  B.  MILNER,  B.A.,  F.G.S.,  F.R.G.S. 
if 

Assistant  Demonstrator  in  Geology  in  the 
University  of  Cambridge 

AND 

GERALD  M.  PART,  B.A.,  F.G.S.,  F.R.G.S. 


CAMBRIDGE 

W.    HEFFER    &    SONS    LTD. 
1916 


MS" 


EARTH 

SCIENCES 

LIBRARY 


INTRODUCTION. 

This  volume  is  intended  to  act  as  a  practical  com- 
panion to  the  standard  Petrological  text-books. 

For  this  reason,  we  have  omitted  all  detailed  descrip- 
tions of  rocks,  minerals,  and  structures. 

We  have  tried  to  indicate  the  great  importance  of 
methodical  procedure  in  the  microscopical  work,  which 
forms  such  an  important  branch  of  Petrology. 

Although  the  usual  University  examinations  do  not 
require  a  knowledge  of  the  preparation  of  rock  slices, 
we  have  made  this  an  important  feature  of  the  book. 

We  have  found  that  very  little  literature  on  this 
subject  is  easily  accessible  to  the  student,  and  we  have, 
therefore,  described  the  processes  involved  in  some 
detail,  in  the  hope  that  the  book  may  be  of  use,  not 
only  to  students,  but  to  those  who  are  starting  research, 
and  to  others  who  have  been  accustomed  in  the  past  to 
send  their  material  to  Germany.  With  this  object  in 
view,  we  have  only  described  methods  which  we  have 
satisfactorily  employed  in  the  Sedgwick  Museum  at 
Cambridge,  and  we  do  not  attempt  to  deal  with  elaborate 
processes  suitable  for  firms  operating  on  a  commercial 
scale. 

In  the  Appendix  we  have  given  the  methods  of 
preparation  of  some  of  the  stains  employed  in  micro- 
scopical work,  so  that  any  chemical  student  may  be 
able,  if  necessary,  to  prepare  the  small  quantities  which 
he  may  require. 


730434 


INTRODUCTION 

We  particularly  desire  to  thank  Prof.  T.  McK. 
Hughes  for  his  kindness  in  placing  at  our  disposal  the 
rock  slice  department  of  the  Sedgwick  Museum,  and 
also  Prof.  T.  G.  Bonney,  Dr.  A.  Harker,  and  other 
members  of  the  staff  for  their  kind  help  and  assistance 
throughout  our  work  at  the  Museum. 

H.  B.  MILNER. 
G.  M.  PART. 

TRINITY  COLLEGE, 

CAMBRIDGE,   1916. 


CONTENTS. 

CHAPTER  I.  PAGE 

Preparation  of  Rock  Slices. 

Apparatus  necessary  —  Rock  cutting  machines  — 
Preparation  of  material — First  grinding — Mounting 
— Second  grinding — Minerals  liable  to  give  trouble 
— Finishing  . .  . .  . .  . .  . .  . .  I 

CHAPTER  II. 

Examination  of  Rock  Slices. 

Method  of  working — Different  stages  discussed — 
Determination  of  Minerals — Becke's  Line — Signifi- 
cance of  Rock  Structures — Mode  of  working  with 
Sedimentary  Rocks — Hybrid  Rocks — Xenoliths  . .  15 

CHAPTER  III. 

Microchemical  Methods  (Staining). 

Uses  of  Staining — Apparatus — Preparation  of  Slice 
— General  method  of  working — Stains  employed 
—  Special  reactions  applicable  to  certain  Minerals  43 

CHAPTER  IV. 

Mounting  of  Sands  and  Crushed  Rock  Material. 

Method  of  Procedure  —  Heavy  liquids  employed  — 
Mounting  —  Crushed  rock  material  —  Schuster's 
method  for  the  determination  of  Felspars  . .  52 

APPENDIX. 

Preparation  of  Stains  ..       60 

INDEX          63 


CHAPTER  I. 
PREPARATION  OF  ROCK  SLICES. 

Apparatus  necessary — Rock  cutting  machines — Preparation  of 

material  —  First  grinding  —  Mounting  —  Second   grinding  — 

Minerals  liable  to  give  trouble — Finishing. 

Rocks  present  such  a  wide  variation  in  texture, 
hardness,  cohesion,  composition,  and  the  mutual  rela- 
tions of  their  constituent  minerals,  that  many  methods 
for  the  preparation  of  rock  slices  have  been  employed 
by  different  workers  to  suit  their  particular  require- 
ments. 

In  this  chapter  we  shall  describe  the  methods  which 
we  have  found  to  be  generally  applicable  to  most  kinds 
of  rock,  though  certain  types  will  necessarily  demand 
special  treatment.  More  particularly  is  this  the  case 
with  some  coarse  varieties  where  the  constituent  minerals 
are  loosely  intergrown. 

Fine  textured  rocks  are  in  general  simpler  to  deal 
with,  their  internal  structure  materially  aiding  the 
production  of  a  thin  slice.  In  general  slices  should  not 
be  more  than  xoVs  inch  thick,  when  quartz  and  felspar, 
if  present  in  the  rock,  will  show  grey  tints  of  the  first 
order  when  examined  in  polarized  light. 

In  the  case  of  very  fine  textured  rocks,  it  may  be 
advisable  to  get  them  even  thinner  than  this  if  possible, 
a  matter  which  requires  much  skill  on  the  part  of  the 
operator. 

I  B 


.2    . 


METHODS  .IN   PRACTICAL   PETROLOGY 


The  possession  of  -apparatus  of  good  quality  is  of 
paramount  -importance,  and  will  save  much  time  and 
trouble  in  the  long  run.  A  simple  form  of  microscope, 
fitted  with  a  low  power  objective  and  polarising  prisms, 
will  be  required  ;  or  a  very  convenient  instrument  can 
easily  be  devised  with  a  strong  pocket  lens,  on  the  lines 
of  the  simple  dissecting  microscope  familiar  to  biologists. 
This  consists  of  a  stage  to  carry  the  slide,  with  a  mirror 
underneath,  and  a  vertical  pillar  supporting  a  horizontal 
arm  at  the  end  of  which  the  lens  and  analyser  are  fitted. 
The  polarizer  is  placed  below  the  stage.  Focussing  is 
effected  by  moving  the  arm  up  or  down  the  pillar. 


Analyser 
fitted  over 
lens 


Glass  plate  covering 
-T,  stage. 

Polariser 


Mirror 


FIG.  i. 


PREPARATION   OF  ROCK  SLICES  3 

A  good  steel  or  bronze  plate  (steel  for  preference), 
I  ft.  square  x  i  inch  thick,  and  two  glass  plates  of  the 
same  dimensions,  will  be  necessary.  The  plates  should 
be  contained  in  wooden  trays  (teak  has  the  advantage 
of  being  waterproof)  not  less  than  15  inches  square, 
with  a  ij  inch  beading.  This  latter  keeps  the  grinding 
material  from  splashing  on  to  neighbouring  plates,  and 
incidentally  this  waste  from  grinding  serves  to  prevent 
the  plates  from  sliding  about  in  the  trays. 

There  are  many  kinds  of  abrasives  used,  but  the  one 
which  seems  to  meet  most  requirements  is  an  artificial 
product,  Carborundum,  supplied  in  many  grades,  of 
which  the  most  generally  useful  are  70  (coarse),  220 
(medium),  and  30  M.M.  (fine). 

The  mountant  used  is  Canada  Balsam,  the  pure 
material,  and  not  a  solution  in  benzene.  The  balsam 
is  a  thick  viscous  liquid  as  supplied,  but  if  heated 
strongly  it  melts  and  sets  hard  on  cooling. 

A  copper  plate  heated  by  gas  or  methylated  spirit 
lamp  is  used  for  the  purpose  of  '  cooking  '  the  balsam  ; 
an  iron  plate  may  be  employed,  but  we  have  found  that 
copper  provides  a  more  uniform  heating  surface  and 
does  not  tend  to  warp  as  some  iron  plates  do. 

Glass  microscope  slides  of  the  usual  size,  3  inches  x 
i  inch,  and  cover  glasses  i  inch  x  J  inch  and  |  inch 
circles,  are  recommended. 

Minor  accessories  include  a  blunt  knife  and  a  pair 
of  forceps. 

Though  not  by  any  means  an  essential  from  the 
student's  point  of  view,  it  is  often  convenient  to  employ 
a  ROCK  CUTTING  APPARATUS.  Many  forms  of  this  have 
been  devised,  of  which  the  most  satisfactory  is  probably 


4  METHODS   IN   PRACTICAL  PETROLOGY 

that  in  which  the  cutting  disc  rotates  in  a  horizontal 
plane. 


FIG.  2. 


PREPARATION  OF  ROCK  SLICES  5 

In  this  form  of  apparatus  (fig.  2),  a  vertical  axis  (A) 
carries  a  circular  mild  steel  cutting  disc  (B)  about  6 
inches  in  diameter  and  -h  inch  thick.  This  is  rotated 
by  a  gut  band  from  the  large  driving  wheel  (C)  worked 
by  the  treadle  (D).  The  rock  to  be  cut  is  clamped  in  a 
vice  (E),  movable  so  that  it  may  be  gradually  fed  up 
to  the  disc  as  the  cut  deepens.  The  disc,  vice,  etc.,  are 
contained  in  a  deep  metal  tray  (F). 

The  lubricant  used  may  be  either  soapy  water  or 
paraffin,  kept  in  a  small  tank  (G)  above  the  cutting 
disc.  The  lubricant  may  be  allowed  to  drip  on  to  the 
disc  by  means  of  a  tap,-  or  may  be  applied  with  a  brush. 

The  disc  is  armed  with  diamond  dust.  To  do  this, 
it  is  first  notched  at  close  intervals  round  its  circumfer- 
ence with  a  blunt  knife.  A  minute  quantity  of  the 
diamond  dust  is  mixed  to  a  thin  paste  with  a  little 
machine  oil  and  applied  to  the  edge  of  the  disc  with  the 
finger  tip.  The  disc  is  then  rotated  against  a  flint  or 
piece  of  agate,  in  order  to  bed  the  diamond  into  the 
metal.  One  such  arming  should  be  sufficient  to  cut  a 
considerable  number  of  rocks.  Care  must  be  taken  not 
to  let  the  disc  become  dry  during  the  process  of  cutting 
or  the  diamond  will  be  immediately  stripped  from  its 
edge. 

Very  good  results  can  also  be  obtained  from  the 
cutting  apparatus  in  which  the  disc  is  carried  on  a 
horizontal  lathe  shaft.  The  general  characters  of  this 
type  of  machine  may  be  gathered  from  the  figure 
(fig.  3).  It  may  be  driven  either  by  a  treadle  or  by  an 
electric  motor ;  the  latter  power  ensures  a  regular 
speed,  but  is  not  so  easily  regulated  to  suit  the  work  in 
hand  as  the  former. 


O  METHODS   IN   PRACTICAL   PETROLOGY 

In  this  apparatus,  diamond  dust  may  be  employed 
for  arming  the  cutting  disc,  as  in  the  machine  previously 


FIG.  3. 

described.  But  by  fitting  a  specially  hardened  steel 
disc,  carborundum  may  be  used ;  this,  mixed  with 
sufficient  water  to  form  a  paste,  is  applied  freely  with  a 
brush  to  the  edge  of  the  disc  during  the  process  of  cut- 
ting. Water,  which  is  contained  in  a  trough  beneath 
the  disc,  is  used  as  the  lubricant. 

The  rock  to  be  cut  is  clamped  in  a  vice  which  operates 
from  a  back  iron  stay,  parallel  with  the  shaft. 

In  both  these  types  of  rock  cutting  machine,  it  is 
most  important  that  there  should  be  no  play  in  the  shaft 
and  that  the  disc  should  run  absolutely  true. 


PREPARATION   OF  ROCK   SLICES  7 

A  carborundum  wheel  will  be  found  very  useful  for 
rough  grinding  and  should  be  interchangeable  with  the 
cutting  disc.  It  must  be  used  with  plenty  of  water. 
These  wheels  are  made  in  a  variety  of  grades.  We  have 
found  a  wheel  of  70  grit,  grade  M,  quite  satisfactory  for 
all  ordinary  purposes. 

Assuming  that  the  student  has  got  his  materials 
ready,  the  first  thing  is  to  obtain  a  suitable  piece  of 
rock  to  work  with.  In  the  majority  of  cases,  a  con- 
venient chip  may  be  obtained  by  flaking  the  specimen 
with  a  hammer. 

When  collecting  in  the  field,  it  is  easy  to  select  chips 
broken  off  a  rock  in  making  a  hand  specimen,  and  two 
or  three  such  chips  should  always  be  kept.  They 
should  be  about  an  inch  square  and  as  thin  as  can  be 
reasonably  obtained.  This  last  condition  will  vary 
with  different  rocks,  so  that  in  dealing  with  some,  it  is 
more  convenient  to  cut  a  piece  with  the  rock  cutting 
apparatus,  if  one  is  available.  Especially  is  this  the 
case  if  the  rock  has  to  be  cut  in  a  special  direction,  as 
when  dealing  with  slates,  when  it  is  often  necessary  to 
cut  a  section  across  the  cleavage. 

In  cases  of  very  friable  or  porous  rocks  we  have 
found  it  advisable  to  boil  the  chip  for  about  five  to  ten 
minutes  in  balsam,  to  which  a  little  shellac  has  been 
added,  before  commencing  operations.  This  can  be 
conveniently  carried  out  in  a  tin  lid. 

This  treatment  will  fill  the  cracks  or  vesicles  in  the 
rock  and  prevent  shattering  during  the  subsequent 
grinding  ;  it  is,  in  addition,  particularly  valuable  during 
the  final  stages  of  the  first  grinding  of  such  rocks,  as  a 
uniform  surface  cannot  in  many  cases  be  obtained 


8  METHODS  IN  PRACTICAL  PETROLOGY 

owing  to  the  tendency  of  existing  holes  and  cracks 
between  the  crystals  to  persist. 

As  examples  of  rocks  which  can  be  advantageously 
treated  in  this  manner,  we  may  mention  : — 

Many    granites    (e.g.    Ballachulish,    N.B.;     Peter- 
head,  Aberdeenshire,  N.B.). 

Luxulyanite  and  other  coarse  Tourmaline  rocks 
(e.g.  Cornwall). 

Coarse  Hornblendic  rocks  (e.g.  Garabal  Hill,  N.B.). 

Eclogites  (e.g.  Bergen,  Norway). 

Shonkinites  (e.g.  Monzoni  district,  Tyrol). 

Coarse  Pyroxenic  rocks  (e.g.  Cran,  Norway). 

Coarse  Peridotites  (e.g.  Harrisite,  Rum,  N.B.). 

"  Trachytes  "  of  the  Melrose  type. 

All  vesicular  volcanic  rocks,  tuffs,  etc. 

Most  Garnetiferous  rocks. 

Phyllites  and  Schists. 

First  Grinding.  For  this,  some  of  the  coarse  carbo- 
rundum (grade  70)  is  spread  on  the  first  (steel)  plate  and 
mixed  with  water. 

The  chip  is  then  ground  down  on  one  side  until  as 
large  an  area  as  possible,  of  a  uniform  character,  is 
obtained.  It  is  then  washed  to  free  it  of  all  traces  of 
carborundum  and  transferred  to  the  second  (medium) 
plate,  where  a  similar  grinding  with  finer  carborundum 
(grade  220)  removes  the  scratches  made  by  the  coarse 
material.  (If  a  carborundum  wheel  is  available,  much 
time  will  be  saved  by  facing  ordinary  hard  rocks  on  it 
preparatory  to  the  first  grinding.  In  many  cases  if  this 
is  done,  the  use  of  the  coarse  plate  may  be  dispensed 
with,  and  the  chip,  after  facing,  should  be  washed  and 
transferred  to  the  second  plate.) 


PREPARATION  OF  ROCK  SLICES  9 

It  is  most  essential  that  the  chip  should  be  well 
washed  to  remove  all  traces  of  the  coarse  powder  before 
using  the  second  plate,  and  the  same  applies  on  trans- 
ferring it  from  that  to  the  fine  plate,  where  the  rock  is 
polished  with  the  30  grade  carborundum,  preparatory  to 
mounting.  (N.B. — Some  workers  prefer  to  use  very 
fine  "  flour  "  emery  (FFFF  grade)  instead  of  carborun- 
dum on  this  third  plate.)  The  finer  the  surface  ob- 
tained, the  better  the  rock  will  adhere  to  the  glass  on 
which  it  is  mounted,  and  to  this  end,  many  of  the  rocks 
cited  above  will  repay  a  final  polishing  with  putty 
powder  and  water,  on  a  green  baize  strip  stretched  on  a 
board.  (N.B. — This  method  may  be  employed  for 
polishing  fossils  or  rock  specimens.) 

The  chip  is  now  washed  and  dried  on  the  hot  plate. 

Mounting.  The  next  operation  is  to  mount  the 
polished  chip  on  a  glass  slide,  and  on  this,  the  success 
of  the  whole  process  depends.  Some  balsam  is  spread 
on  a  glass  slide  and  heated  on  the  copper  plate.  It  is 
sufficiently  '  cooked  '  when  a  thread  drawn  out  between 
the  points  of  a  pair  of  forceps,  becomes  brittle  when 
cold.  Insufficiently  cooked  balsam  will  absorb  carbo- 
rundum during  the  second  grinding.  Overcooked  balsam 
is  too  brittle  and  will  break  away,  thus  exposing  the  edges 
of  the  rock  chip  which  it  is  there  to  protect. 

When  the  balsam  is  just  right  for  mounting,  the  rock 
chip,  which  has  also  been  heating  meanwhile,  is  placed, 
polished  surface  downwards,  on  the  slide,  and  firmly 
pressed  down,  care  being  taken  to  exclude  air  bubbles. 
If  any  of  these  remain  the  slide  must  be  re-heated  and 
the  chip  worked  about  until  the  bubbles  are  pressed  out. 
When  cold,  the  second  grinding  may  be  proceeded  with. 


10  METHODS  IN   PRACTICAL  PETROLOGY 

Second  Grinding.    The  chip  is  now  ground  on  the 
coarse  plate  until  it  is  about  I  m.m.  thick,  the  glass  slide 


FIG.  4. 

being  used  as  a  convenient  holder.  It  is  then  well  washed 
and  transferred  to  the  medium  plate.  (N.B. — When  a 
rock-cutting  machine  of  the  horizontal  type  is  available 
the  chip  may  be  cut  very  thin  by  holding  the  glass 
slide,  to  which  it  is  attached,  just  below  the  level  of  the 
cutting  disc  (fig.  4).  In  this  way  it  is  possible  to  leave 
only  a  thin  layer  of  rock  adhering  to  the  glass,  and,  after 
washing,  this  may  be  dealt  with  on  the  medium  plate, 
thus  avoiding  the  often  tedious  coarse  grinding  on  the 


PREPARATION   OF   ROCK   SLICES  II 

first  plate.  It  must  be  understood,  however,  that  this 
use  of  the  rock-cutting  machine  requires  great  skill  on 
the  part  of  the  operator,  and  should  not  be  attempted 
until  the  student  has  had  considerable  practice  in  its 
ordinary  manipulation.) 

If  care  is  taken,  grinding  may  be  continued  on  the 
second  plate  until  quartz  or  felspars,  if  present  in  the 
rock,  show  first  order  reds  and  yellows  with  polarised 
light. 

At  this  point  it  is  advisable  to  wash  and  transfer 
to  the  fine  plate.  It  will  sometimes  be  found  that  the 
balsam  tends  to  come  away  from  the  edges  of  the  rock 
a  little  before  this  stage  is  reached.  In  such  cases  the 
slide  should  be  transferred  as  soon  as  this  happens ;  or 
if  the  balsam  behaves  very  badly  in  this  respect  (showing 
that  it  has  been  overcooked)  the  slide  should  be  washed 
and  dried,  and  a  little  freshly  cooked  balsam  applied 
round  the  edges  of  the  chip. 

Care  must  be  taken  to  avoid  unequal  pressure  on 
different  parts  of  the  slice  during  the  final  stages,  and 
any  tendency  to  thickness  in  any  particular  part  may 
be  checked  by  grinding  that  part  on  the  edge  of  the 
plate  until  a  uniform  thickness  is  once  more  obtained. 

The  thinness  of  the  slice  ultimately  attained  will 
depend  very  largely  on  the  skill  of  the  operator.  It  is 
advisable  not  to  use  too  much  water  or  carborundum  on 
the  fine  plate,  and  to  check  carefully  the  progress  of  the 
operations  with  the  microscope. 

When  the  slice  is  as  thin  as  desired,  it  is  washed  and 
dried  ready  for  finishing. 

Certain  constituent  minerals  of  many  rocks  render 
the  final  stages  of  the  grinding  a  somewhat  delicate 


12  METHODS   IN   PRACTICAL   PETROLOGY 

operation,  so  that  the  student  has  sometimes  to  choose 
between  having  a  slightly  thick  slice  and  sacrificing  some 
of  its  minerals.  Very  good  examples  of  this  are  Garneti- 
ferous  Phyllites,  where  sections  thin  enough  to  show  the 
composition  and  structure  of  the  main  body  of  the  rock 
will  usually  be  devoid  of  the  garnets,  which  may  drop 
out  even  before  the  section  is  transparent.  Or  again, 
Felspar-Mica  Schists,  where  the  soft  mica  occurring 
interstitially  between  the  felspar  laths  will  be  ground 
away  before  the  felspar  is  reasonably  thin.  The  follow- 
ing minerals  are  always  liable  to  give  trouble  in  this 
way  : — 

Hornblende  and  Augite. 

Garnet. 

Olivine. 

Leucite. 

Corundum, 

and  occasionally  quartz,  if  present  in  large  grains 
embedded  in  a  soft  matrix. 

Much  trouble  of  this  kind  may  be  avoided  by  suit- 
able '  cooking '  during  the  first  grinding,  and  by  even 
pressure  during  the  final  stages,  but  in  some  cases  it  is 
almost  impossible  to  avoid  losses  of  this  nature. 

Frequently,  glassy  rocks  such  as  Tachylites  and 
Variolites  are  hard  to  grind  really  thin,  but  provided  a 
section  is  sufficiently  transparent  for  examination  by 
ordinary  light,  this  will  not  be  of  extreme  importance. 

In  the  case  of  rocks  which  do  not  contain  evident 
quartz  or  felspar,  other  minerals  will  have  to  be  used  as 
indices  of  thickness.  Augite  and  Olivine  should  not 
exceed  a  thickness  giving  higher  colours  than  those  of 
the  second  order,  while  the  calcite  of  limestones  should 


PREPARATION   OF  ROCK  SLICES  13 

show  the  delicate  colours  of  the  third  and  fourth  orders 
in  polarised  light. 

Finishing  o!  Slide.  Having  carried  the  process  of  the 
preparation  of  the  rock  slice  as  far  as  the  end  of  the 
second  grinding,  it  now  remains  to  complete  the  slide  for 
examination.  To  this  end  it  is  first  dried  on  the  hot 
plate,  but  not  sufficiently  heated  to  loosen  the  balsam 
which  cements  the  rock  to  the  glass.  It  is  then  allowed 
to  cool.  On  another  glass  slide,  some  fresh  balsam  is 
cooked  as  previously  described,  and  poured  over  the 
rock  slice.  A  cover  glass  of  suitable  size  is  then  placed 
on  top  of  the  balsam  and  the  whole  transferred  to  the 
hot  plate.  As  the  temperature  rises,  the  balsam  becomes 
less  viscuous,  and  the  cover  glass  is  gently  pressed  down 
on  to  the  slice,  at  the  same  time  expelling  any  air  bubbles 
present.  At  this  point  the  surplus  balsam  can  be  re- 
moved with  a  knife  while  still  fluid  and  the  slide  allowed 
to  cool. 

Probably  many  variations  on  this  method  can  be 
employed ;  for  instance  the  balsam  might  be  heated  on 
the  cover  glass,  but  we  have  found  the  above  method 
generally  satisfactory,  as  the  rock  remains  firmly  fixed 
to  the  glass  throughout  the  operations,  and  there  is  little 
tendency  for  the  constituent  minerals  to  disintegrate. 

Many  workers  make  a  practice  of  transferring  the 
slice,  when  finished,  to  a  new  glass  slide.  To  do  this, 
the  original  slide  is  heated  on  the  hot  plate  with  some 
fresh  balsam  ;  at  the  same  time  some  more  balsam  is 
heated  on  a  clean  glass  slide  in  the  usual  manner.  When 
the  rock  slice  has  been  heated  sufficiently  to  melt  the 
balsam  which  cements  the  rock  to  the  glass  on  which  it 
has  been  ground,  a  cover  glass  may  be  placed  on  the 


14  METHODS   IN   PRACTICAL  PETROLOGY 

top  of  it  and  then  worked  off  on  to  the  new  glass  slide, 
with  the  rock  section  adhering  to  its  under-surface. 

Owing  to  the  tendency  of  many  rocks  to  break  up 
during  this  last  operation,  the  process  is  not  to  be 
recommended,  except  for  the  purpose  of  remounting 
broken  slides. 

Microscope  slides  thus  prepared  may  be  conveniently 
cleaned  by  soaking  in  methylated  spirit,  when  the 
surplace  balsam  washes  off  as  a  milky  white  precipitate. 
The  slide  is  then  dried  with  a  clean  cloth,  and  is  ready  for 
use. 


CHAPTER  II. 

EXAMINATION  OF  ROCK  SLICES. 

Method  of  working — Different  stages  discussed — Determination 

of  Minerals — Becke's  Line — Significance  of  Rock  Structures — 

Mode  of  working  with  Sedimentary  Rocks — Hybrid  Rocks — 

Xenoliths. 

In  this  chapter  we  propose  to  give  an  outline  of  the 
methods  which  we  have  employed  in  the  examination 
and  determination  of  rocks  in  thin  slices. 

Haphazard  methods  often  lead  to  erroneous  conclusions, 
and  much  time  and  trouble  may  be  saved  by  a  methodical 
process. 

At  the  outset  a  great  deal  can  be  learnt  by  holding 
the  slide  up  to  the  light  and  examining  it  with  a  pocket 
lens. 

If  the  hand  specimen  of  the  rock  is  available,  it  should 
be  carefully  studied  at  the  same  time,  for  macroscopical 
details  of  structure  and  composition. 

The  following  mode  of  procedure  should  now  be 
adopted,  though  with  practice  the  student  will  -perform 
the  preliminary  stages  instinctively.  (It  will  be  seen 
that  the  process  involved  is  one  of  elimination.) 

i.  Determination  as  to  whether  the  rock  is  Igneous 
or  Sedimentary.1 

i  If  sedimentary,  see  p.  38. 
15 


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EXAMINATION   OF  ROCK  SLICES  17 

2.  If  Igneous,  whether  Plutonic,  Hypabyssal,  Vol- 

canic or  Metamorphic.1 

3.  Whether   Acid,    Intermediate,    Basic    or   Ultra- 

basic. 

4.  Determination  of  Constituent  Minerals  in  order 

of    crystallisation    if    possible,     i.e.     Rosen- 
busch's  order  of  decreasing  basicity.2 
[Frequently   deviations   from   this   order   may   be 

observed,  but  in  any  case  it  is  convenient  to  determine 

the  minerals  as  follows  : — 

(A)  Accessories. 

(B)  Ferromagnesian  minerals, 
(c)  Felspars  and  Felspathoids. 
(D)  Quartz  (if  present).] 

5.  From  the  results  of  (4)  whether  the  rock  belongs 

to  the  Alkaline  or  Calcic  branch,  or  is  inter- 
mediate between  these  two.3 

6.  Note  structures  and  any  special  characteristics.4 

7.  Inference  as  to  probable  nature  of  the  rock. 

We  will  now  proceed  to  discuss  some  special  points 
which  may  arise  in  each  of  the  above  stages ;  in  most 
cases  the  method  holds  good,  but  it  must  be  clearly 
understood  from  the  outset  that  it  is  merely  a  means  to 


i  In  considering  metamorphic  rocks,  it  must  be  remembered 
that  they  may  originally  be  of  either  igneous  or  sedimentary 
origin.  The  metamorphic  character  of  a  rock  will  usually  be 
apparent  from  stages  (4)  and  (6),  and  therefore  any  rock  not 
definitely  sedimentary  is  best  treated  on  the  lines  suggested 
above. 

sHarker.  Natural  History  of  Igneous  Rocks,  p.  180  ;  also 
Petrology  for  Students,  1908,  pp.  23  and  24. 

3  Marker.     Natural  History  of  Igneous  Rocks,  Ch.  4. 

4  ibid.,  Ch.  ii  ;  also  Michel  Levy.     Memoir e  sur  les  divers 
modes  de  structure  des  roches  eruptives,  1875. 


l8  METHODS   IN   PRACTICAL   PETROLOGY 

an  end,  and  that  rocks  do  not  necessarily  always  accom- 
modate themselves  to  such  an  arrangement. 

Thus  it  is  sometimes  difficult  to  decide  in  the  case  of 
the  Charnockite  series  of  Southern  India,1  to  what 
extent  a  given  rock  may  be  described  as  plutonic  or 
metamorphic. 

Again,  some  of  the  so-called  "  Syenites  "  of  Charn- 
wood  Forest,  Leicestershire,  appear  to  be  the  common 
meeting  point  of  Syenites  (as  ordinarily  understood), 
Diorites  and  Granophyres  (see  also  under  "  Hybrids," 
p.  41). 

1.  Igneous  or  sedimentary  origin  of  the  rock.    In 
general  the  igneous  or  sedimentary  origin  of  a  rock  will 
be  obvious  at  a  glance,  but  in  some  cases  confusion  may 
arise.     Careful  examination  of  a  slice  will,  however, 
nearly  always  show  a  clastic  or  fragmental  structure, 
and  usually  the  presence  of  some  cementing  material 
between  the  grains,  in  the  case  of  sedimentary  rocks. 
Limestones  are  easily  determined  if  crystalline  calcite 
or  dolomite  are  present,  or  if  the  remains  of  organisms 
make  up  part  of  the  rock.     Calcite  and  dolomite  will 
show    their    characteristic    birefringence    colours   with 
polarised  light,  and  generally  grey  colours  of  the  first 
order,  characteristic  of  pyrogenetic  quartz  and  felspar 
will  be  absent  from  the  slice. 

Holocrystalline,  porphyritic  or  glassy  structures,  so 
characteristic  of  different  groups  of  igneous  rocks,  are 
not  found  in  unaltered  sedimentary  material. 

2.  Plutonic  Rocks.    Plutonic  rocks  are  characterised 
by  a  moderate  to  coarse  holocrystalline   structure  in 

i  Holland.     Mem,  Geol.  Sur.  India,  Charnockite  Series,  1900. 


EXAMINATION  OF  ROCK  SLICES  IQ 

which  the  main  mass  of  the  rock  consists  of  an  aggregate 
of  mutually  interfering  crystals.  As  a  rule,  only  the 
accessory  minerals,  which  are  the  first  products  of 
crystallisation,  show  good  idiomorphic  outlines. 

Porphyritic  structure  is  not  common,  except  in  the 
more  acid  members  of  the  series. 

Glassy  structures  do  not  occur. 

Hypabyssal  Rocks.  Many  authors  object  to  this 
division  of  rocks,  and  regard  them  as  dyke  phases  of  the 
Plutonic  group,  but  there  are  so  many  rock  types  which 
do  not  conveniently  fall  into  either  the  Plutonic  or  the 
Volcanic  groups,  that  they  are  best  treated  as  a  separate 
division  intermediate  between  these  two. 

Hypabyssal  rocks  are  in  general  characterised  by  a 
holocrystalline  and  porphyritic  structure.  In  many 
types,  the  ground  mass  of  the  rock  is  very  fine  grained 
and  may  even  be  glassy,  though  some  of  the  larger 
masses  approach  the  Plutonic  group  in  structure,  e.g. 
Dolerites. 

Porphyritic  structure  involves  the  presence  of  some 
of  the  constituent  minerals  in  two  generations,  and  this 
will  necessitate  a  slight  modification  in  the  method  of 
procedure  outlined  in  (4)  (seep.  22).  The  phenocrysts 
and  the  ground  mass  should  be  dealt  with  separately, 
the  order  suggested  in  stage  (4)  being  applied  to  the 
minerals  of  each  group  in  turn,  though  it  must  be 
remembered  that  usually  no  particular  order  of  crystal- 
lisation can  be  distinguished  for  the  phenocrysts. 

Volcanic  Rocks.  These  rocks  may  be  holocrystalline, 
but  commonly  contain  more  or  less  glass.  The  main 
characteristic  of  those  which  are  not  wholly  glassy,  is  the 
porphyritic  structure  described  above,  though  a  few 


20  METHODS   IN   PRACTICAL  PETROLOGY 

non-porphyritic  types  occur.  Certain  minerals  such  as 
hornblende,  biotite,  and  muscovite,  characteristic  of  the 
plutonic,  hypabyssal  and  metamorphic  rocks,  are  not 
usually  found.  Hornblende  and  biotite,  if  present,  are 
almost  always  greatly  corroded ;  muscovite  will  only 
occur  as  an  alteration  product  in  decomposed  rocks. 

Many  lavas  are  characterised  by  the  presence  of 
amygdales,  often  filled  with  calcite,  zeolites  and  other 
secondary  minerals,  and  by  a  parallel  orientation  of  the 
predominant  minerals,  due  to  flow. 

Certain  of  the  common  types  of  volcanic  rocks  occur 
also  as  intrusions  (sills  and  dykes),  and  in  the  absence  of 
field  evidence,  cannot  be  distinguished  from  effusive 
lavas  (e.g.  Basalts,  and  the  Trachytes  and  related 
alkaline  instrusions  of  Carboniferous  age  in  Southern 
Scotland).1 

[Pyroclastic  rocks  will  be  dealt  with  under  the 
sedimentary  division  (p.  40).] 

Metamorphic  Rocks.  These  frequently  show  traces 
of  crushing  (e.g.  strain  shadows  in  quartz,  or  granular 
structure),  and  banding,  due  either  to  the  thermal 
metamorphism  of  sediments,  whose  original  stratifica- 
tion has  not  been  obliterated,  or  to  the  crushing  of 
igneous  rocks  subsequent  to  their  consolidation  (dyna- 
mic metamorphism).2 


iMcRobert.     Q.J.G.S.,  1914,  Vol  LXX,  p.  303  et  seq. 

a  It  should  be  noted  that  many  granites  which  are  of  un- 
doubted primary  igneous  origin  (e.g.  Laurentian  of  Canada) 
show  a  gneissic  structure  due  to  flow,  but  having  been  formed 
under  conditions  of  great  pressure,  they  exhibit  many  of  the 
characters  of  metamorphic  rocks. 


EXAMINATION   OF   ROCK   SLICES  21 

Metamorphic  rocks  are  also  in  many  cases  character- 
ised by  the  development  of  certain  minerals  which 
require  special  mineralising  agents1  for  their  formation, 
and  by  others  which  are  not  usually  of  magmatic  origin. 

3.  Acid  Rocks.  Acid  rocks  are  characterised  by 
the  presence  of  a  quantity  of  free  quartz  and  alkali 
felspars. 

Quartz  sometimes  occurs  in  quite  basic  types,  but 
never  in  any  quantity,  and  in  such  cases,  the  basic 
felspars  will  indicate  the  nature  of  the  rock  (e.g.,  Quartz 
Gabbro,  Carrock  Fell,2  and  Quartz  Dolerite,  Whin  Sill3); 
Felspathoids  never  occur. 

Intermediate  Rocks.  These  are  characterised  by 
predominant  felspar ;  quartz,  at  the  most,  is  only  an 
accessory.  Ferromagnesian  minerals  are  important 
in  these  types,  chiefly  augite  and  hornblende.  In  the 
absence  of  quartz,  felspathoids  may  be  present. 

Basic  Rocks.  The  chief  feature  of  these  rocks  is  the 
presence  of  lime-soda  felspars  and  pyroxenes  as  pre- 
dominant constituents.  Olivine  is  important  in  many 
types,  while  quartz  is  typically  absent. 

In  alkaline  magmas,  alkali  felspars  and  felspathoids 
may  partially  take  the  place  of  the  lime-soda  felspars 
(e.g.  Essexites,  Monteregian  Hills,  Quebec).4 

Iron  ore  minerals,  often  titaniferous,  are  important 
accessories. 


1  Harker.     Nat.  Hist.  Ig.  Rocks,  Ch.  12. 

2  Harker.     Q.J.G.S.,  1894,  Vol.  50,  p.  311. 

3  Teall.     Q.J.G.S.,  1884,  Vol  XL.,  pp.  640-657. 

4  Internal.   Geol.  Congress,    1913,    Guide  No.   3.    Excurs. 
Monteregian  Hills. 


22  METHODS  IN   PRACTICAL  PETROLOGY 

Ultra-basic  Rocks.  Felspar,  if  present,  is  of  a  basic 
type,  but  sometimes  is  wanting  altogether.  Except  in 
the  anorthite-peridotites  (e.g.  Allivalite,  Rum,  N.B.)1 
felspar  is  subordinate  to  the  ferromagnesian  minerals, 
the  chief  of  which  is  olivine. 

In  alkaline  types,  felspathoids  may  be  important 
constituents  (e.g.  Alnoite,  Alno,  Sweden)2  and  in  many 
cases  spinellid  minerals  are  common  accessories. 

4.  Determination  of  Minerals.  We  do  not  propose 
to  discuss  here  the  general  properties  of  rock  forming 
minerals  as  the  student  will  find  these  fully  set  out  in 
the  standard  works  on  the  subject.3  We  shall,  however, 
describe  the  general  mode  of  procedure  in  determining 
minerals  and  note  certain  cases  where  confusion  may 
easily  arise.  We  give,  under  the  various  headings, 
tables  containing  some  useful  data  for  dealing  with 
these. 

General  method  of  procedure  for  determination  of 
minerals  in  a  rock  slice. 

(a).  WITH  TRANSMITTED  WHITE  LIGHT.  Note  colour, 
crystalline  form,  cleavage,  clearness  of  outline  and 
visibility  of  surface,  the  two  latter  giving  an  idea  of  the 
refractive  index.4  This  may  be  higher  or  lower  than 
that  of  the  balsam  in  which  the  slice  is  mounted ;  we 
may  note  here  that  balsam  has  approximately  the  same 


1  Mem.  Geol.  Sur.     Geol.  of  Small  Isles,  1908,  Ch.  7. 

2  Harker.     Petrology  for  Students,  1908,  p.  160. 

3  Iddings.     Rock  Minerals,  1906  ;    also  Miers.    Mineralogy, 
1902. 

4  Sorby.     On  a  new  method  of  studying  the  Optical  Proper- 
ties of  Minerals,  York  Pol.  Soc.,  1878. 


EXAMINATION   OF  ROCK  SLICES  23 

refractive  index  as  quartz  (1-55).  In  consequence,  clear 
quartz  is  nearly  invisible  in  ordinary  light,  while  garnet 
(refractive  index  1-7-1-8)  exhibits  a  very  distinct  outline 
combined  with  a  '  rough  '  surface. 

Opaque  minerals  should  be  examined  with  reflected 
light ;  this  may  be  done  by  shielding  the  light  from  the 
mirror  below  the  microscope  stage,  and  reflecting  light 
on  to  the  slide  by  means  of  a  white  card  held  above  it, 
providing  the  light  is  strong  enough. 


Table  of  Mean  Refractive  Indices. 

Fluor 

•43 

Muscovite 

1-58 

Hypersthene   1 

•70 

Sodalite 

•48 

Biotite 

1-59 

Barkevicite 

•70 

Hauyne 

•49 

Dolomite 

1-59 

Zoisite 

70 

Analcime 

•49 

Topaz 

1-62 

Arfvedsonite 

•71 

Leucite 

•51 

Tremolite 

1-62 

Idocrase 

•72 

Orthoclase 

1-52 

Tourmaline 

•63 

Magnesite 

•72 

Cancrinite 

1-52 

Actinolite 

•63 

Chloritoid 

•72 

Microcline 

1-53 

Wollastonite 

•63 

Kyanite 

•72 

Albite 

1-53 

Melilite 

•63 

Augite 

•72 

Nepheline 

1-54 

Aragonite 

•63 

Staurolite 

•74 

Oligoclase 

1-54 

Andalusite 

•64 

Chalybite 

•75 

Kaolin 

•54 

Apatite 

•64 

Garnet     1-76- 

•81 

Cordierite 

•54 

Hornblende 

•65 

Corundum 

•76 

Quartz 

•55 

Fosterite 

•66 

Epidote 

•76 

Serpentine 

•55 

Sillimanite 

•67 

Aegirine 

•78 

Andesine 

•56 

Enstatite 

•67 

Sphene 

•89 

Labradorite 

1-57 

Axinite 

•68 

Zircon 

•95 

Calcite 

1-57 

Olivine 

•68 

Cassiterite       2 

•00 

Chlorite 

1-58 

Diallage 

•68 

Perovskite       2 

•38 

Anorthite 

1-58 

Diopside 

•68 

Anatase           2 

•53 

Scapolite 

1-58 

Riebeckite 

•69     Rutile              2 

•76 

METHODS   IN   PRACTICAL  PETROLOGY 


(b).      POLARISER  INSERTED  BELOW  THE  STAGE.   Note 

pleochroism.     The  following  minerals  when  present  in 
rock  slices  are  usually  pleochroic  : — 


Intensely  Pleochroic  : 


Moderately  Pleochroic 


Slightly  Pleochroic  : 


Biotite. 
Hornblende. 
Riebeckite. 
Hypersthene. 
Epidote  (varies). 
Tourmaline. 
Chloritoid. 

Kyanite  (if  coloured). 
Chlorite. 

Enstatite  (varies). 
Sphene. 
Staurolite. 
Idocrase. 

Corundum  (if  coloured). 
Aegirine. 
Titaniferous  varieties  of  Augite. 


(c).  ANALYSER  INSERTED  (CROSSED  NICOLS).  If 
the  mineral  remains  isotropic  on  rotation  of  the  micro- 
scope stage  though  90°,  it  may  be  either  a  glass,  a  cubic 
mineral,  or  a  section  which  has  been  accidentally  cut 
perpendicular  to  an  optic  axis. 

If  the  mineral  is  birefringent,  the  birefringence 
colours  should  be  noted,  and  also  the  extinction  angle 
on  cleavages  or  crystal  faces  (if  developed)  should  be 
determined. 

It  will  now  be  convenient  to  deal  with  certain 
minerals  which  owing  to  their  somewhat  similar  physical 
properties,  may  easily  be  confused.  Conforming  to  the 


EXAMINATION   OF  ROCK   SLICES 


Minerals 

arranged  in  order  of  Birefringence. 

UNIAXIAL  POSITIVE.                    UNIAXIAL  NEGATIVE. 

Pennine 

•003 

Idocrase 

•001 

Quartz 
Zircon 
Cassiterite 

•009 
•062 
•099 

Apatite 
Nepheline 
Melilite 

•004 
•005 
•005 

Rutile 

•287 

Corundum 

•008 

Tourmaline 

•017 

Scapolite 
Cancrinite 

-021 
•028 

Anatase 

•061 

Calcite 

•172 

Dolomite 

•189 

BIAXIAL  POSITIVE.                      BIAXIAL  NEGATIVE. 

Zoisite 

•006 

Orthoclase 

•006 

Albite 

•008 

Microcline 

•007 

Enstatite 

•009 

Andesine 

•007 

Topaz 
Staurolite 

•009 
•010 

Oligoclase 
Kaolin 

•008 
•008 

Chloritoid 

•015 

Cordierite 

•008 

Augite 
Sillimanite 

•021 
•021 

Labradorite 
Axinite 

•008 
•009 

Diallage 
Diopside 
Olivine 

•024 
•030 
•036 

Andalusite 
Serpentine 
Anorthite 

•Oil 
•Oil 
•012 

Sphene 
Brookite 

•121 

•160 

Hypersthene 
Wollastonite 

•013 
•014 

Kyanite 
Actinolite 

•016 
•025 

Tremolite 

•028 

Muscovite 

•038 

Epidote 
Biotite 

•040 
•044 

Hornblende 

•932- 

Aragonite 

•156 

26  METHODS  IN   PRACTICAL  PETROLOGY 

original  order  in  stage  (4)  (p.  17),  we  have  firstly  the 
Accessory  Minerals. 

DISTINCTION  BETWEEN  IRON  ORE  AND  OTHER  OPAQUE 
MINERALS.  With  the  exception  of  HAEMATITE  and 
CHROMITE,  which  are  translucent  in  exceedingly  thin 
sections,  all  the  iron  ore  minerals  are  opaque  ;  they  are 
generally  distinguished  by  their  colour  in  reflected  light, 
and  in  some  cases  by  their  crystalline  form  or  character- 
istic decomposition  products.  On  p.  27  we  give  a  table 
for  the  determination  of  these  and  certain  other  opaque 
minerals. 

APATITE  may  be  mistaken  for  TOPAZ  or  colourless 
TOURMALINE,  all  of  which  give  straight  extinction  and 
have  somewhat  similar  refractive  indices.  Apatite 
commonly  shows  an  hexagonal  outline  in  cross-section, 
tourmaline  occurs  in  irregular  or  acicular  forms,  while 
topaz  is  often  granular.  Tourmaline  has  high  birefrin- 
gence (in  the  coloured  varieties  this  may  be  masked  by 
the  absorption),  topaz  has  a  birefringence  as  high  as 
quartz,  while  that  of  apatite  is  lower.  Apatite  occurring 
in  fine  needles  may  be  confused  with  RUTILE,  which  is 
commonly  found  included  in  other  minerals  (e.g. 
Biotite)1;  but  the  very  high  refractive  index  and 
double  refraction  of  the  rutile  serve  to  distinguish  it. 

There  is  a  possibility  of  a  confusion  between  CASSI- 
TERITE  and  SPHENE  ;  the  latter  has  a  lower  refractive 
index  and  a  higher  birefringence  than  the  former,  and 
is  often  slightly  pleochroic,  which  is  not  characteristic 
of  cassiterite.  Sphene  usually  occurs  in  crystals  giving  a 
diamond  shaped  section,  cassiterite  in  tetragonal  prisms. 

i  Mennell.     Manual  of  Petrology,  1913,  p.  53. 


EXAMINATION   OF  ROCK  SLICES 


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28  METHODS   IN   PRACTICAL  PETROLOGY 

Of  the  Metamorphic  minerals  CORDIERITE  may  be 
easily  overlooked  owing  to  its  resemblance  to  QUARTZ 
in  refractive  index  and  birefringence.  Cordierite  when 
coloured  is  pleochroic,  but  if  it  is  colourless,  it  can  only 
be  distinguished  by  its  biaxial  interference  figure,  using 
convergent  polarised  light.1  A  characteristic  alteration 
product  is  the  irregular  aggregate  of  chlorite  and  mus- 
covite  known  as  '  Finite.'  2 

ANDALUSITE  and  SILLIMANITE  bear  considerable 
resemblance  to  one  another,  but  the  latter  has  a  higher 
refractive  index  and  birefringence.  If  coloured,  Anda- 
lusite  is  pleochroic  ;  it  commonly  occurs  in  the  form 
"CHIASTOLITE,"  in  which  included  carbonaceous  material 
tends  to  group  itself  in  particular  directions  in  the  crystal. 

WOLLASTONITE  3  may  be  distinguished  from  the 
colourless  AUGITE  with  which  it  occurs  in  many  impure 
metamorphosed  limestones,  by  its  lower  refractive 
index,  birefringence  and  extinction  angle. 

Some  of  the  alteration  products  of  certain  common 
minerals  are  liable  to  cause  confusion  at  times,  such 
as  KAOLIN,  WHITE  MICA,  and  CALCITE,  which  occur  in 
the  turbid  aggregates  which  discolour  many  felspars, 
together  with  EPIDOTE,  ZOISITE  and  CHLORITE.  In 
these  aggregates  they  can  as  a  rule  only  be  distinguished 
by  chemical  means. 

Some  varieties  of  CHLORITE,  and  SERPENTINE,  show 
a  similar  deep  blue  birefringence  tint,  but  Chlorite  can 
be  distinguished  by  its  greenish  colour  in  ordinary  light, 


1  See  Miers.     Mineralogy,  1902. 

2  Mem.  Geol.  Sur.,  351  and  358,  1907,  p.  52. 

3  A  pyroxene,  but  more  conveniently  dealt  with  as  a  meta- 
morphic  "  accessory." 


EXAMINATION   OF  ROCK   SLICES  2Q 

and  by  its  slight  pleochroism  with  simple  polarised 
light. 

EPIDOTE  may  be  mistaken  for  AUGITE  or  OLIVINE  ; 
it  is,  however,  generally  pleochroic,  and  has  a  higher 
refractive  index  and  birefringence,  and  usually  occurs 
in  irregular  granular  aggregates  of  a  dull  greenish-yellow 
colour. 

ZOISITE  has  a  somewhat  similar  appearance  to 
Epidote,  but  is  paler  in  colour,,  and  has  a  low  birefrin- 
gence. 

CANCRINITE,  a  decomposition  product  of  Nepheline, 
may  be  mistaken  for  White  Mica,  but  has  a  less  perfect 
cleavage  and  shows  birefringence  colours  of  a  lower  order. 

Ferromagnesian  Minerals.  BIOTITE  is  often  con- 
fused with  HORNBLENDE,  especially  the  deep  brown 
varieties  of  the  latter  (Barkevicite),  but  its  lower  re- 
fractive index,  basal  cleavage  and  straight  extinction 
serve  to  distinguish  it. 

Hornblende  having  an  extinction  angle  normally 
of  about  12°  and  a  cleavage  angle  of  approximately  55°, 
is  readily  distinguishable  from  AUGITE,  in  which  the 
extinction  angle  is  commonly  40°  or  more,  and  the 
cleavage  angle  is  87°.  Augite  rarely  shows  pleochroism 
to  any  marked  extent. 

AEGIRINE  may  be  mistaken  for  green  Hornblende  in 
longitudinal  sections,  but  the  marked  pleochroism  and 
larger  extinction  angle  of  the  latter  will  in  most  cases  be 
sufficient  to  distinguish  them. 

DISTINCTION  BETWEEN  THE  RHOMBIC  AND  MONO- 
CLINIC  PYROXENES.  Rhombic  pyroxenes  are  charac- 
terised by  straight  extinction,  and  the  varieties  rich  in 
iron  (HYPERSTHENE)  show  a  moderate  pleochroism ; 


30  METHODS  IN  PRACTICAL  PETROLOGY 

they  also  have  a  lower  birefringence  than  the  mono- 
clinic  forms. 

OLIVINE  is  readily  confused  with  AUGITE,  but  the 
latter 's  cleavage,  slightly  lower  refractive  index  and 
large  extinction  angle,  and  the  former's  straight  extinc- 
tion, irregular  fracture  and  usual  alteration  to  serpentine 
along  the  cracks,  will  serve  to  distinguish  them. 

If  developed,  the  crystal  shapes  of  all  these  ferro- 
magnesian  minerals  are  very  characteristic. 

The  Felspars.  For  some  reason  students  nearly 
always  appear  to  have  difficulty  in  determining  the 
nature  of  the  felspars  present  in  a  rock  slice.  In  practice, 
this  difficulty  is  more  apparent  than  real. 

MICROCLINE  is  at  once  recognisable  by  its  character- 
istic "  cross-hatching  "  z  in  polarised  light. 

ORTHOCLASE  frequently  shows  twinning  on  the 
Carlsbad  law,  and  this  can  be  recognised  by  the  alternat- 
ing extinction  shown  by  the  two  halves  of  the  crystal  on 
rotation.  The  pure  and  colourless  variety  is  known  as 
'  ADULARIA/  and  that  met  with  in  many  volcanic  rocks  is 
'  SANIDINE/  distinguished  from  common  orthoclase,  by 
its  transparency  and  vitreous  lustre. 

Orthoclase  is  frequently  turbid  owing  to.  decomposi- 
tion products  (see  p.  28)  ;  it  is  very  commonly  inter- 
grown  with  the  alkaline  plagioclase  felspars,  to  form 
'  PERTHITE.' 

There  remains  the  isomorphous  series  of  Plagioclase 
Felspars. 

These,  except  in  certain  metamorphic  rocks,  in- 
variably show  a  repeated  lamellar  twinning,  usually  on 

1  Miers.     Mineralogy,  1902,  p.  460. 


EXAMINATION  OF  ROCK  SLICES  31 

the  Albite  law,  but  they  may  also  show  twinning  on  the 
Pericline  law,  perpendicular  to  the  Albite  lamellae. 

This  results  in  a  "  cross-hatched  "  appearance  distin- 
guishable from  that  of  Microcline  by  the  clearness  of  the 
st nations,  which  in  the  latter  mineral  are  usually  "spindle 
shaped  "  and  somewhat  indistinct.  In  a  few  cases  only 
the  pericline  twinning  is  present  (e.g.  certain  gabbros). 

Determination  of  Plagioclase  Felspars.1  To  deter- 
mine a  felspar  by  means  of  its  extinction  angle  measured 
from  the  albite  lamellae,  select  in  the  slice,  crystals  in 
which  the  two  sets  of  lamellae  give  equal  extinction 
angles  on  opposite  sides  of  a  cross  wire  (fig.  5). 


»•*.* ' 


FIG.  5. 

Plagioclase  crystal  parallel  with  cross-wire  (B)  and  in 
positions  of  extinction  for  the  two  sets  of  lamellae  (A  and  C), 
showing  equal  extinction  angles  a,  ft,  on  each  side  of  the 
cross-wire. 


1  See  also  Harker.      Petrology  for  Students,  1908,  pp.  10-14. 


METHODS   IN   PRACTICAL  PETROLOGY 


Measure  a  number  of  such  crystals  and  select  the 
maximum  angle  obtained.  In  the  case  of  MICROLITES, 
it  is  necessary  to  measure  the  extinction  angles  from 
their  long  axes.  Obtain  a  series  of  several  measure- 
ments and  choose  the  highest  reading  determined. 
Below  we  give  a  table  of  the  characteristic  extinction 
angles  of  the  plagioclase  felspars  for  use  with  the  above 
methods. 


EXTINCTION  ANGLE 

EXTINCTION  ANGLE 

MEASURED    FROM 

MEASURED   FROM 

FELSPAR. 

THE   ALBITE 

THE   LONG   AXES 

LAMELLAE. 

OF   MICROLITES. 

Albite 

6°-  16° 

10°  -  20° 

Oligoclase 
Oligoclase  (basic) 
Andesine 

0°-    5° 
6°  -16° 
16°  -22° 

0°-     7° 

8°  -  20° 

Labradorite 

27°  -  45° 

30°  -  42° 

Bytownite 
Anorthite 

45°  -  50° 
50°  and  over 

49°  -  56° 

58°  -  64° 

It  will  be  seen  from  the  above  table  that  there  is  an 
ambiguity  between  Albite  and  Andesine  in  both  cases. 
But  examination  of  the  refractive  indices  of  these  felspars 
with  reference  to  quartz  or  balsam  (see  p.  33),  will  do 
much  to  decide  the  point.  Further,  Albite  and  Ande- 
sine usually  occur  in  rocks  o&quite  different  character, 
and  reference  to  the  other  constituent  minerals  will  often 
assist  the  student  in  avoiding  this  difficulty. 

In  cases  where  the  extinction  angles  measured  on 
the  albite  lamellae  exceed  40°,  there  will  be  an  am- 
biguity between  the  Labradorite  and  the  more  basic 


EXAMINATION  OF  ROCK  SLICES  33 

felspars  consequent  on  there  being  two  principal  direc- 
tions of  extinction  in  a  crystal  section.  This,  though 
of  theoretical  importance,  will  not  cause  any  difficulty 
in  practice,  as  we  have  found  that,  apart  from  other 
considerations,  the  birefringence  and  refractive  index  of 
Anorthite,  combined  with  its  confinement  to  particular 
type  of  rocks  in  which  Labradorite  does  not  normally 
occur,  suffice  for  its  determination.  On  the  other  hand,  if 
it  is  desirable  to  distinguish  between  these  two  directions 
of  extinction,  use  must  be  made  of  a  quartz  wedge  or 
mica  plate,  for  which  the  student  should  consult  such 
works  as  Miers' '  Mineralogy,' x  etc. 

It  is  not  safe  to  assume,  in  the  case  of  basic  felspars, 
that  the  lamellar  twinning  present  is  necessarily  developed 
on  the  Albite  law,  and  care  should  be  taken  to  check  this 
with  the  cleavage  traces  in  the  crystals  used  for  measure- 
ment. 

In  really  doubtful  cases,  the  rock  should  be  crushed, 
and  the  felspars  determined  by  Schuster's  method  as 
described  in  Chapter  IV. 

Becke's  method  for  determining  relative  refractive 
indices.  This  will  prove  of  great  assistance  in  dealing 
with  the  felspars  and  many  other  rock-forming  minerals. 
If  the  junction  of  two  substances  such  as  quartz  and 
Canada  balsam  be  examined  under  the  microscope  with 
ordinary  light,  it  will  be  seen  that  in  this  case  it  is  very 
indistinct,  owing  to  the  similarity  of  their  respective 
refractive  indices.  It  is  found  that  the  greater  the 
difference  in  refractive  index  between  any  two  sub- 
stances so  examined,  the  more  distinct  the  junction 
becomes. 

i  Ch.  VII. 


34  METHODS   IN   PRACTICAL  PETROLOGY 

If  the  objective  be  slightly  raised  so  as  to  throw  the 
junction  out  of  focus,  it  will  be  observed  that  a  white 
line  passes  from  the  substance  having  the  lower  refrac- 
tive index  into  that  having  the  higher.  If  the  objective 
be  lowered,  the  reverse  takes  place,  and  the  greater  the 
difference  between  the  refractive  indices  the  more  marked 
is  this  effect. 

For  example,  it  will  be  seen  from  the  table  (p.  23) 
that  albite  and  oligoclase  have  lower  refractive  indices 
than  quartz  (or  balsam),  and  consequently,  any  felspar 
having  a  higher  refractive  index  than  these  latter,  must 
be  andesine,  or  a  more  basic  variety. 

The  student  will  learn  by  experience  that,  in  general, 
acid  rocks  are  characterised  by  acid  felspars  and  basic 
rocks  by  basic  felspars,  and  that  a  rapid  examination  of 
the  refractive  index  of  a  felspar  will  go  a  long  way  to 
determine  its  nature,  without  recourse  to  more  elaborate 
optical  methods. 

Felspathoids.  These  contain  a  lower  percentage  of 
silica  than  the  felspars,  and  are  not  formed  from  magmas 
in  which  there  is  sufficient  silica  to  produce  the  latter. 
Consequently  they  need  not  be  looked  for  in  any  rock 
containing  pyrogenetic  quartz. 

NEPHELINE  (a  dull  variety  has  been  distinguished  as 
EL^OLITE)  is  sometimes  confused  with  Orthoclase,  but 
has  a  refractive  index  as  high  as  oligoclase  and  can  be 
distinguished  in  addition,  by  its  lower  birefringence, 
absence  of  twinning,  and  crystal  shape  if  developed.  In 
many  rocks,  however,  nepheline  is  interstitial,  and  can 
often  only  be  determined  by  microchemical  methods 
(see  p.  47). 

LEUCITE,   usually  in  icosetetrahedra,   often   shows 


EXAMINATION   OF  ROCK   SLICES  35 

weak  anomalous  birefringence,  and  zoning  by  inclusions, 
and  may  be  distinguished  from  nepheline  by  the  latter's 
higher  refractive  index  and  crystalline  form,  if  developed. 

MELILITE  often  shows  lath-shaped  sections  resembl- 
ing felspars  ;  it  has,  however,  a  higher  refractive  index, 
and  much  weaker  birefringence.  In  addition  it  may 
show  striations  perpendicular  to  the  long  axis  ("  peg 
structure  "Y  and  is  commonly  discoloured.  The  above 
characters  also  serve  to  distinguish  it  from  nepheline, 
which  it  resembles  in  birefringence. 

Of  the  remaining  felspathoids,  NOSEAN  is  easily 
identified  by  its  characteristic  zoning,  HAUYNE  and 
SODALITE  are  often  bluish  in  thin  section,  the  colour 
being  usually  irregularly  distributed  in  the  case  of 
Hauyne,  and  very  pale  in  that  of  Sodalite.  For  micro- 
chemical  methods  applicable  to  all  the  above  fels- 
pathoids, see  Chapter  III. 

Quartz.  In  many  metamorphic  rocks,  it  is  hard  to 
differentiate  between  quartz  and  clear  recrystallized 
felspar  grains.  In  such  cases  the  characteristic  twinning 
of  plagioclase  felspars  is  frequently  not  developed,  and 
staining  methods  may  be  resorted  to,  if  examination  of 
the  refractive  indices  gives  no  conclusive  result.  It 
may  be  noted  that  quartz  never  occurs  with  felspathoids 
and  seldom  with  olivine,  and  also  that  the  mineral  never 
shows  twinning  in  rock  slices. 

5.  Having  determined  the  mineral  constitution  of 
the  rock,  the  student  will  be  in  a  position  to  decide 
whether  it  belongs  to  the  Alkaline  or  Calcic  group,  or 
occupies  a  position  Intermediate  between  these  two 

i  Hatch.     Petrology,  1909,  p.  102. 


36  METHODS   IN   PRACTICAL  PETROLOGY 

(Monzonite  types).1  In  the  Alkaline  group  we  find  that 
the  quartz  is  generally  confined  to  the  acid  rocks,  that 
alkali  felspars  occur  more  or  less  throughout  the  series, 
while  the  felspathoids  are  common  in  the  more  basic 
varieties,  and  may  entirely  take  the  place  of  felspar. 
The  ferromagnesian  minerals  are  often  soda-bearing, 
e.g.  Arfvedsonite,  Riebeckite,  Aegirine,  etc. 

In  the  Calcic  group,  quartz  ranges  from  acid  to 
quite  basic  rocks,  felspathoids  are  absent,  and  alkali 
felspars  are  generally  confined  to  acid  members.  Ferro- 
magnesian minerals  are  typically  non-alkaline  varieties. 

In  the  monzonite  magmas,  alkaline  and  calcic 
felspars  occur  in  approximately  equal  amounts. 

6.  Rock  Structures  and  their  significance.  In  the 
examination  of  a  rock  slice,  the  student  should  bear  in 
mind  that  one  of  the  objects  of  Petrology  is  to  assist  in 
unravelling  the  geological  history  of  the  earth,  and  that 
his  object  in  examining  a  slice  should  not  be  merely  to 
find  out  whether  certain  minerals  occur  in  his  particular 
rock,  but  to  interpret  as  far  as  the  limitations  of  micro- 
scopical methods  allow,  the  history  of  that  rock,  both 
during  and  after  consolidation.  In  this  connection 
three  points  are  of  great  importance  : — 

(1)  The  particular  association  of  minerals  present  in 

a  rock  is  only  one  of  the  possible  expressions  of 
its  chemical  composition. 

(2)  This   particular   mineral   association   will   vary 

according  to  the  physical  conditions  (e.g. 
pressure)  under  which  consolidation  has  taken 
place. 

i  Harker.  Petrology  for  Students,  1908,  pp.  50,  56  and  57  ; 
also  Hatch,  Petrology,  pp.  183  and  198. 


EXAMINATION   OF  ROCK   SLICES  37 

(3)  The  original  character  of  the  minerals  present 
may  be  partially  or  entirely  changed,  during 
the  rock's  subsequent  history. 

From  the  above,  it  will  be  seen  that  the  structure 
of  a  rock  is,  at  least,  equal  in  importance  to  its  minera- 
logical  composition. 

It  is  not  our  province  here  to  describe  the  characters 
of  various  rock  structures,  as  these  are  dealt  with  very 
fuDy  in  the  standard  works  on  Petrology.1  It  will 
suffice  to  say  that  in  general  the  student  will  find  that 
certain  rocks  are  characterised  by  particular  types  of 
structure. 

7.  Inference  as  to  probable  nature  of  the  rock.  From 
an  intelligent  use  of  all  the  foregoing  considerations  the 
student  should  now  be  in  a  position  to  determine,  within 
limits,  the  nature  of  the  rock  that  he  is  dealing  with. 

It  must  be  borne  in  mind  that  rock  types  grade 
imperceptibly  into  one  another,  and  that  occasionally 
rocks  will  be  found  that  are  intermediate  in  this  respect. 
On  this  account  the  classification  of  rocks  as  at  present 
employed  leaves  much  to  be  desired,2  and  one  gets  cases 
where  existing  rock  names  do  not  suit  the  specimens 
under  consideration.3 


1  Harker.      Petrology  for  Students,  1908  ;   also  Teall.  British 
Petrography,  1888.     Hatch.  Petrology,  1909,  and  Iddings.  Igneous 
Rocks,  Vols.  i  and  2,  1909. 

2  Harker.     Nat.  Hist.  Ig.  Rocks,  Ch.  15  ;   also  Petrology  for 
Students,   1908,  p.   20 ;    also  Iddings.     Igneous  Rocks,  Vol.   i, 
p.  334  et  seq.  ;  and  Lake  and  Rastall.  Text  book  of  Geology,  1910, 
p.  235  et  seq.  ;    and  Mennell.  Manual  of  Petrology,  1913,  P-  81 
et  seq. 

3  McRobert.     Q.J.G.S.,  Vol.  70,  1914,  p.  315. 


38  METHODS   IN   PRACTICAL   PETROLOGY 

Method  of  Working  if  the  rock  is  Sedimentary.    If 

the  rock  is  of  sedimentary  origin,  it  will  be  necessary  to 
decide  whether  it  has  been  formed  : — 

(1)  By  denudation  of  land  masses. 

(2)  By  organic  agencies. 

(3)  From  the  products  of  volcanic  activity,  or  by  any 

combination  of  (i),  (2)  and  (3). 

We  will  consider  these  in  order  : — 

i.  Sedimentary  rocks  formed  by  the  denudation  of 
land  masses,  can  be  divided  into  two  main  groups  (a) 
ARENACEOUS,  and  (b)  ARGILLACEOUS. 

(a)  The  chief  points  to  determine  in  Arenaceous 
rocks  are  the  minerals  which  comprise  the  grains,  the 
cementing  material  and  the  structure.     Careful  study 
of  the  grains  may  indicate  sources  of  origin  ;  it  must  be 
remembered,  however,  that  in  general  only  the  more 
stable  minerals  of  igneous  and  metamorphic  origin  will 
be  found  (see  p.  53).     The  cementing  material  may  be 
ferruginous,  calcareous,  silicious  or  argillaceous.    Under 
the  head  of  "  structure,"  we  include  such  points  as 
coarseness  or  fineness  of  the  grains,  as  well  as  their 
shapes  and  mutual  relations. 

(b)  Argillaceous   rocks.     This   group   includes  the 
finer  sediments  such  as  clays,  mudstones,  shales  and 
slates.     Rock  sections  of  such  sediments  as  shales  and 
slates  often  present  considerable  difficulty  in  complete 
determination,  owing  to  their  general  fine  texture  and 
the  prevalence  of  decomposition  products  such  as  seri- 
citic  mica,  chlorite,  etc.     They  frequently  show  a  fissile 
structure  due  to  a  parallelism  of  their  constituents  ;  this 
may  be  the  outcome  of  original  bedding,  or  of  the  effects 
of  compression,  i.e.  cleavage  in  slates. 


EXAMINATION  OF  ROCK  SLICES  39 

The  high  power  objective  of  the  microscope  will 
prove  of  great  use  in  determining  these  rocks. 

2.  Rocks  due  mainly  to  organic  agencies.  These 
may  be  either  (a)  CALCAREOUS,  (b)  SILICIOUS,  or  (c) 
CARBONACEOUS. 

(a)  Calcareous.    These  are  rarely  of  detrital  origin, 
but  are  commonly  composed  of  comminuted  fragments 
of  the  hard  parts  of  such  organisms  as  mollusca,  echi- 
nodermata,  actinozoa,  or  foraminfera,  set  in  a  matrix  of 
fine  calcareous  mud. 

Organic  structures  originally  composed  of  Aragonite 
(e.g.  some  mollusca),  are  usually  replaced  by  a  granular 
aggregate  of  calcite. 

The  calcareous  matrix  is  often  recrystallised  by 
simple  pressure,  but  this  does  not  necessarily  imply  any 
great  degree  of  metamorphism. 

Special  types  of  concretionary  structures  such  as 
oolitic  or  pisolitic  should  be  noted. 

Another  important  characteristic  of  many  limestones 
is  the  replacement  of  all  or  part  of  the  calcite  by  DOLO- 
MITE, x  and  more  rarely  by  CHALYBITE  (ironstones)  or 
SILICA  (cherts).  Dolomite  may  usually  be  distinguished 
from  calcite  by  its  tendency  to  crystal  habit  (rhombo- 
hedra),  and  absence  of  lamellar  twinning,  but  in  other 
cases  chemical  tests  are  the  only  sure  means  of  discrimi- 
nation (seep.  49). 

(b)  Silicious.    The   silicification   of   limestones   to 
form  chert  may  be  due  either  to  infiltration  or  to  the 
presence   of    such   silicious   organisms   as   Sponges2   or 

1  Skeats.     Q.J.G.S.,  LXI,  1905,  pp.  97-138  and  references 
there  cited. 

2  Hinde.     Phil.  Trans.  Roy.  Soc.,  1885. 


40  METHODS   IN   PRACTICAL   PETROLOGY 

Radiolaria,  the  remains  of  which  should  be  looked  for  in 
such  cases.1 

(c)  Carbonaceous.  Specially  prepared  thin  sections 
of  coal  will  show  traces  of  vegetable  structures  such  as 
stems,  fructification,  etc.,  for  the  study  of  which  we  must 
refer  the  student  to  standard  Palaeobotanical  works.2 

3.  Pyroclastic  Rocks.  A  microscopical  study  of 
pyroclastic  rocks  may  throw  much  light  on  the  nature 
of  the  volcanic  outburst  which  gave  rise  to  them. 

They  are,  in  general,  made  up  of  shattered  fragments 
of  pre-existing  types  of  rock,  together  with  varying 
amounts  of  usually  decomposed  glass  and  other  fragments 
of  volcanic  origin. 

It  should  be  the  student's  aim  to  determine  the 
nature  of  the  fragments  and  of  the  volcanic  matrix 
which  encloses  them. 

In  other  instances  the  rock  will  be  made  up  wholly  of 
volcanic  material.  In  such  cases,  this  material  should 
be  treated  as  described  when  dealing  with  volcanic  rocks 
(p.  19),  and  according  to  its  nature  may  be  designated  as 
rhyolitic,  trachytic,  andesitic,  or  basaltic  tuff. 

Silicification3  may  be  looked  for  in  rhyolitic  tuffs. 

Many  tuffs  were  originally  deposited  under  the  sea, 
and  the  finer  varieties  often  show  a  conspicuous  bedding  ; 
secondary  cleavage  is  quite  commonly  observed  in  those 
varieties  which  have  been  subjected  to  pressure.  Where 
such  a  cleavage  is  well  marked,  secondary  minerals, 


1  Howard    Fox.     Radiolarian    Cherts   of  Cornwall,    Trans. 
Roy.  Geol.  Soc.  Corn.,  Vol.  12. 

2  Scott.     Studies  in  Fossil  Botany,  1900. 

3  Harker  and  Marr.     Q.J.G.S.,  1891,  p.  303  et  seq. 


EXAMINATION   OF   ROCK   SLICES  4! 

which  have  been  formed  as  a  result  of  these  stresses, 
should  be  looked  for. 

Hybrid  Rocks.1  Hybrid  rocks  may  be  expected  to 
show  many  of  the  characters  of  metamorphic  rocks,  but 
their  heterogenous  origin  will  make  itself  apparent  in 
unusual  structures  and  peculiar  mineral  associations. 
These  rocks,  from  the  very  nature  of  their  composition, 
will  not  allow  of  the  methodical  microscopic  treatment 
which  we  have  outlined  for  normal  rocks.  In  general,  a 
very  complete  knowledge  is  required  of  the  circumstances 
of  any  particular  case,  both  as  regards  the  types  of  rocks 
which  have  contributed  to  the  mixture,  and  the  latter's 
mode  of  occurrence  in  the  field. 

Consequently  a  microscopical  examination  of  such 
rocks,  unless  supplemented  by  this  knowledge,  must 
necessarily  be  somewhat  arbitrary. 

In  the  absence  of  circumstantial  evidence,  the 
student  is  advised  to  make  a  very  thorough  determina- 
tion of  the  minerals  present,  and,  if  possible,  to  separate 
those  of  the  original  rock  from  those  of  the  one  which  has 
incorporated  it. 

In  many  cases  superimposed  structures  will  render 
this  impossible,  and  it  is  then  that  microscopical  exami- 
nation alone  breaks  down.  But  if  the  field  evidence  is 
available,  and  the  types  of  rock  which  have  taken  part  in 
the  mixture  can  be  studied  both  macro-  and  micro- 
scopically, the  student  should  be  in  a  position  to  formu- 
late an  opinion  as  to  how  far  intermixing  has  proceeded 
in  the  particular  case  he  is  considering. 

i  Harker.  Nat.  Hist.  Ig.  Rocks,  1909,  ch.  14.  ;  also  Mem. 
Geol.  Sur.,  Tertiary  Ig.  Rocks  Skye,  1904,  Ch.  u  and  p.  231  ;  and 
Mem.  Geol.  Sur.,  Sheet  60,  Scotland,  Ch.  9 ;  and  Mennell. 
Manual  of  Petrology,  1913,  p.  209. 


42  METHODS   IN   PRACTICAL  PETROLOGY 

Closely  allied  to  this  question  of  Hybridism  is  the 
presence  in  numerous  igneous  rocks  of  inclusions  (Xeno- 
lithsY ;  these  may  be  accidental  and  derived  from 
extraneous  sources,  in  which  case  they  differ  from  the 
hybrid  rocks  only  in  degree,  or  they  may  be  closely 
bound  up  with  the  formation  of  the  rock  which  contains 
them  (cognate  xenoliths). 

In  the  first  case  a  study  of  such  inclusions  may  be  of 
assistance  in  considering  more  advanced  stages  of 
admixture,  while  the  occurrence  of  cognate  xenoliths 
may  throw  light  on  the  affinities  of  the  rock  in  which 
they  are  found.  Unlike  the  true  hybrids,  such  cognate 
xenoliths  are  in  nearly  all  cases  amenable  to  the  treat- 
ment which  we  have  suggested  for  normal  igneous  rocks. 


i  Lacroix.      Les  enclaves  des  roches  volcaniques,  1893  •'    and 
Harker.  Nat.  Hist.  Ig.  Rocks,  p.  346  et  seq. 


CHAPTER  III. 
mCROCHEMICAL    METHODS    (STAINING). 


Uses   of   Staining — Apparatus — Preparation   of   Slide — General 
•ocedure — Stains     em 
applicable  to  certain 


Method    of    Procedure — Stains     employed — Special    reactions 

.  Minerals. 


In  certain  branches  of  organic  microscopy,  staining 
methods,  which  depend  on  microchemical  reactions,  are 
of  great  importance,  and  somewhat  analogous  methods 
have  been  extended  to  petrological  work,  with  many 
satisfactory  results. 

Such  methods  are  used  as  special  or  confirmatory 
tests  for  certain  rock-forming  minerals,  which  are  difficult 
of  determination  in  rock  slices  by  ordinary  microscopical 
means,  either  on  account  of  their  mode  of  occurrence  or 
resemblance  to  other  minerals.  In  the  following  para- 
graphs we  propose  to  deal  with  the  simplest  methods  of 
staining  which  can  be  rapidly  applied  to  certain  minerals 
in  a  rock  slice. 

In  most  cases  these  microchemical  tests  are  modifica- 
tions of  reactions  employed  when  dealing  with  minerals 
in  bulk,  the  mode  of  application  being  changed  to  suit 
the  conditions  of  microscopical  examination.  It  should 
be  noted  that  the  study  of  these  reactions  constitutes  in 
itself  a  wide  field  for  research,  and  we  can  only  outline  a 
few  of  the  more  useful  cases.1 


i  See  H.  Behrens.     A  Manual  of  Microchemical  Analysis, 
London,   1894. 

43 


44  METHODS   IN   PRACTICAL   PETROLOGY 

It  will  be  convenient  to  deal  with  the  matter  under 
three  heads : — 

1.  Apparatus  and  preparation  of  slice  for  chemical 

tests. 

2.  General  method  of  procedure  with  regard  to  re- 

agents employed. 

3.  Special  reactions  applicable  to  certain  minerals. 

i.  Beyond  the  necessary  chemicals  involved,  little 
extra  apparatus  is  required  for  these  microchemical 
tests,  though  a  capillary  tube  or  pipette  for  transference 
of  the  reagent  to  the  mineral  under  examination,  or  a 
glass  rod  for  applying  it  drop  by  drop,  together  with  a 
diamond  cutter,  will  be  found  useful  accessories. 

The  ordinary  petrological  microscope  will  be  quite 
efficient  for  our  purpose,  though  a  cheaper  pattern  or 
the  simple  type  recommended  for  use  in  the  manu- 
facture of  rock  slices  (see  p.  2),  will  be  an  advantage  ; 
continued  exposure  to  the  presence  of  mineral  acids,  even 
in  minute  quantities,  will  not  improve  the  condition  of  a 
more  expensive  instrument. 

Slight  modifications  in  the  use  of  Canada  balsam  may 
be  necessary,  such  as  its  employment  dissolved  in  carbon 
di-sulphide  or  ether,  which  on  evaporation,  leave  it  hard. 

If  the  staining  of  a  particular  mineral  is  to  be  a 
permanent  feature  of  the  slide,  great  care  must  be 
exercised  in  carrying  out  the  operations  involved. 

In  the  case  of  slices  already  finished,  it  will  be  first 
of  all  necessary  to  uncover  a  portion  of  the  rock.  This 
may  be  done  in  several  ways.  Either  the  slide  may  be 
warmed  until  the  balsam  is  soft,  when  the  cover  glass 
may  be  partially  removed,  or  a  portion  of  the  cover  glass 
may  be  perforated  with  a  diamond  cutter,  over  that  part 


MICROCHEMICAL  METHODS    (STAINING)  45 

of  the  slice  to  be  experimented  upon.  A  neater  method 
than  the  latter  is  to  coat  the  cover  glass  with  fluid  bees- 
wax, and  when  just  cool,  to  scratch  a  circle  through  the 
latter  with  a  sharp  point.  Apply  a  small  quantity  of 
hydrofluoric  acid  to  this,  which  will  speedily  etch  through 
the  glass. 

In  the  case  of  slices  in  course  of  preparation,  when 
they  have  reached  the  end  of  the  second  grinding  (see 
p.  10),  they  may  be  covered  with  a  thick  layer  of  balsam 
dissolved  in  ether,  and  a  hole  made  over  the  part  of  the 
slice  to  be  treated.  The  mounting  of  the  cover  glass 
should  be  delayed  until  the  staining  operations  have 
been  completed. 

If  the  subsequent  test  demands  the  use  of  hydro- 
fluoric acid,  a  glass  cover  slip  for  the  protection  of  that 
part  of  the  slice  not  to  be  experimented  on,  will  obviously 
be  useless.  In  such  a  case  the  method  of  covering  the 
whole  slice  with  a  layer  of  balsam,  as  outlined  above, 
should  be  employed. 

The  part  of  the  rock  exposed,  is  cleaned  with  methy- 
lated spirit  or  benzene,  to  free  it  from  balsam. 

In  the  cases  where  balsam  is  employed  to  protect 
portions  of  the  slice,  care  must  obviously  be  exercised  in 
the  use  of  benzene  or  other  solvents  for  cleaning  purposes. 
In  this  connection  a  camel-hair  brush  may  be  used  with 
advantage. 

2.  Before  minerals  can  be  stained,  it  is  usually 
necessary  to  gelatinise  them  with  acids  capable  of 
effecting  their  solution.  The  etching  must  be  purely 
superficial  and  the  reaction  must  not  be  allowed  to  go 
on  for  longer  than  absolutely  necessary.  The  success  of 
the  subsequent  staining  operations  depends  on  the 


46  METHODS   IN   PRACTICAL  PETROLOGY 

complete  removal  of  the  acid  from  the  gelatinous  film 
formed.  This  is  usually  achieved  by  washing  with  a 
solution  of  ammonia,  care  being  taken  not  to  wash  away 
the  gelatinous  material.  In  many  cases  the  ammonia 
not  only  effects  the  removal  of  the  acid  by  neutralisation, 
but  materially  aids  the  staining. 

When  washed  and  dried,  the  section  is  placed  in  a  dish 
containing  the  necessary  stain. 

The  stains  generally  employed  are  Fuchsine  (Magenta), 
Malachite  green,  Congo  red,  Aniline  blue  and  Methylene 
blue.1  Fuchsine  has  the  disadvantage  of  not  being  per- 
manent in  the  presence  of  Canada  balsam,  and,  moreover, 
it  fades  on  prolonged  exposure  to  light.  Malachite  green 
is  a  more  conspicuous  colour,  and  is  permanent ;  the  other 
dyes  mentioned  are  also  quite  satisfactory  in  this  respect. 

When  the  staining  is  complete,  the  slide  is  washed ; 
it  will  be  found  on  examination  through  the  microscope, 
that  all  minerals  which  have  gelatinised  with  the  particu- 
lar reagent  employed,  will  appear  coloured.  If  the 
staining  has  not  been  carried  far  enough,  the  process  can 
be  repeated. 

3.  We  will  now  describe  the  application  of  the  above 
methods  to  certain  minerals. 

The  following  minerals  gelatinise  with  Hydrochloric 
acid  and  take  the  stain  : — 


Lime-scapolite. 

Lazurite. 

Nepheline. 

Anorthite. 

Sodalite. 

Olivine. 

Melilite. 

Chlorite. 

Hauyne. 

Serpentine. 

Nosean. 

Zeolites. 

i  See  Appendix. 

MICROCHEMICAL  METHODS   (STAINING)  47 

Distinctive  tests  for  some  of  the  above  minerals. 

These  should  be  observed  microscopically  throughout 
the  entire  process. 

LIME  SCAPOLITE  and  SODALITE  when  treated  with  a 
solution  of  silver  nitrate  in  hydrofluoric  acid,  form  a 
gelatinous  film  which  turns  brown  when  acted  on  by  a 
photographic  developer  such  as  pyro-soda.1 

SODALITE  dissolves,  without  gelatinising,  in  nitric 
acid,  and  crystals  of  sodium  chloride  are  formed  on 
evaporation  of  the  solution. 

NEPHELINE  will  take  the  stain,  using  hydrochloric 
acid,  but  its  presence  is  only  certain  if  the  stained  mineral 
shows  characteristic  crystal  outline,  or  by  an  elimination 
of  other  possibilities. 

MELILITE,  if  gelatinised  with  hydrochloric  acid,  con- 
taining drops  of  sulphuric  acid,  will  show  crystals  of 
gypsum. 

HAUYNE  gelatinises  with  hydrochloric  acid  forming 
crystals  of  gypsum. 

NOSEAN  generally  occurs  in  rocks  in  association  with 
sodalite.  On  treatment  with  1:3  acetic  acid,  containing 
a  little  barium  chloride  in  solution,  nosean  will  be  clouded 
with  a  precipitate  of  barum  sulphate,  and  sodalite,  if 
present,  will  remain  clear. 

LAZURITE.  If  this  is  treated  with  a  solution  of  silver 
nitrate  in  hydrofluoric  acid,  a  gelatinous  film  is  formed, 
which  turns  black  owing  to  deposition  of  sulphide  of 
silver.2 

1  A  few  drops  of  the  normal  strength  used  in  photographic 
work   will    suffice.     See    Bothamley.    Manual    of  Photography 
(Ilford,  London),  p.  67. 

2  For  this  and  the  above  Lime-Scapolite  reaction,  see  also 
Weinschenk  (Clark),  Petrographic  Methods,  1912,  p.  174. 


48  METHODS   IN   PRACTICAL   PETROLOGY 

OLIVINE,  particularly  the  varieties  rich  in  iron, 
gelatinises  with  hydrochloric  and  sulphuric  acids,  slowly 
with  cold,  and  rapidly  with  hot  acids. 

CHLORITE,  SERPENTINE,  ZEOLITES,  etc.,  are  often  the 
cause  of  the  cloudy  aggregates  of  decomposition  pro- 
ducts which  discolour  many  rock-forming  minerals. 
These  can,  in  general,  be  completely  removed  by  pro- 
longed digestion  in  hydrochloric  acid. 

General  methods  involving  the  use  of  hydrofluoric 
acid.  The  following  is  useful  as  a  distinctive  test  for 
FELSPAR  when  present  with  QUARTZ  in  fine  recrystallised 
aggregates,  as  is  often  the  case  in  metamorphic  rocks.1 

The  section  exposed  is  treated  for  one  minute  with 
hydrofluoric  acid,  when  it  is  found  that  felspar  is  super- 
ficially altered,  becoming  cloudy,  and  quartz,  though 
acted  upon,  remains  clear.  It  is  possible  to  accentuate 
this  difference  by  staining.  When  the  etching  is  com- 
plete, the  slide  is  washed  and  dried,  and  treated  with 
aniline  blue  for  ten  minutes.  It  is  then  washed  with 
water,  after  which  it  is  treated  with  ethyl  alcohol  of 
increasing  strength  to  remove  water  from  the  gelatinous 
material.  After  a  final  immersion  in  absolute  alcohol, 
the  slice  should  be  moistened  with  turpentine  or  benzene, 
and  finally  covered  with  Canada  balsam  dissolved  in 
benzene  (see  p.  55).  A  cover  glass  may  then  be  placed 
on  the  top  of  the  balsam  and  gently  pressed  down,  and 
the  slice  should  then  be  put  away  to  dry. 

This  somewhat  complicated  operation  is  rendered 


i  T.  Harada.  Das  Luganer  Eruptivgebiet,  Neues  Jahrb., 
B.B.,  II.  (1883),  p.  14 ;  also  F.  Becke.  Unterscheidung  von 
Quarz  und  Feldspath  in  Dunnschliffen  mittelst  Farbung,  Tscher- 
mak's  Min.,  u.  Petr.,  Mitt.,  Vienna,  X  (1889),  p.  90. 


MICROCHEMICAL  METHODS   (STAINING)  49 

necessary  by  the  desirability  of  not  disturbing  the 
gelatinous  material  by  treatment  with  liquids  of  greatly 
different  density. 

It  is  possible  by  these  methods  to  distinguish  the 
felspars  by  the  depth  of  colour  which  they  exhibit 
after  staining.  But  a  great  deal  depends  on  the  skill 
of  the  operator  in  timing  the  reaction.  In  general, 
the  lime  felspars  show  a  deep  blue,  the  soda  varieties 
being  paler  in  colour  and  orthoclase  only  very  slightly 
affected. 

APATITE.  When  this  is  treated  with  hot  concentrated 
nitric  acid,  followed  by  a  solution  of  ammonium 
molybdate,  the  characteristic  ammonium  phospho- 
molybdate  colour  (canary  yellow)  is  obtained. 

CARBONATES.  Carbonates  are  often  hard  to  dis- 
tinguish by  ordinary  microscopical  examination,  and  in 
these  cases  microchemical  tests  are  of  extreme  value. 
We  will  deal  first  with  the  separation  of  CALCITE  and 
DOLOMITE. 

Calcite  dissolves  in  cold  hydrochloric  acid,  dolomite 
being  unaffected.  The  latter  is  however  soluble  in  hot 
acid.  They  may  be  further  separated  by  Linck's l 
method  : — 20  c.c.  of  ammonium  phosphate  are  dissolved 
in  30  c.c.  of  dilute  acetic  acid.  Treatment  with  this 
solution  will  completely  dissolve  calcite ;  dolomite  is 
practically  unaffected,  though  it  may  be  covered  with 
crystal  aggregates  of  magnesium  ammonium  phosphate. 

Lemberg's  method  2: — To  30  parts  of  water,  2  parts  of 


1  G.  Linke.      Abh.  zur.  geol.  Spezialkarte  von  Elsass-Loth- 
ringen,  III.,  1884,  17. 

2  J.  Lemberg.      Zeitschr  d.  deutsch  geol.  Besell,  XL.,  1888, 
357-359- 

E 


50  METHODS   IN   PRACTICAL  PETROLOGY 

dry  aluminium  chloride  and  3  parts  of  log  wood  (haema- 
toxylin)  are  added.  On  boiling,  a  deep  violet  solution  is 
formed,  which  is  filtered  off  and  allowed  to  cool.  The 
slice  is  treated  with  this  stain  for  about  five  minutes,  and 
then  carefully  washed  with  water.  The  calcite  will  be 
found  to  be  coloured  violet,  while  dolomite  remains  un- 
changed. 

Heeger's  method  I: — Both  these  last  two  methods  of 
separation  fail  when  the  carbonates  occur  as  fine 
interstitial  matter.  In  such  a  case  the  following  method 
may  be  employed  with  advantage. 

It  consists  of  treating  the  rock  slice  with  a  few  c.c. 
of  decinormal  hydrochloric  acid,  containing  in  solution  a 
little  potassium  ferricyanide.  Calcite  will  give  violent 
effervescence,  and  on  washing,  after  a  few  seconds,  will 
be  found  to  be  coloured  blue.  This  is  due  to  the  fact 
that  nearly  all  calcite  occurring  in  this  manner  contains 
more  or  less  iron.  Dolomite  is  practically  unaffected. 

SEPARATION  OF  CALCITE  AND  ARAGONITE.  The 
application  of  a  cold  solution  of  ferrous  ammonium 
sulphate  to  calcite  and  aragonite  causes  the  latter  to 
turn  olive  green  in  colour,  whilst  the  former  is  un- 
affected. 

These  minerals  may  also  be  separated  by  Panebianco's2 
method  : — Powdered  calcite  heated  with  a  pure  dilute 
cobalt  nitrate  solution,  turns  blue  after  one  minute's 
boiling,  whilst  aragonite  turns  lilac.  Continued  heating 
determines  the  lavender  blue  colouration  of  calcite,  and 
the  violet  colouration  of  aragonite. 

1  W.  Heeger.     Centralbl.  f.  Min.,  etc.,  Stuttgart,  1913,  44-51- 

2  G.    Panebianco.      Revista   di   min.   crist.   Ital.    XXVIII. 
(1902),  5. 


MICROCHEMICAL  METHODS   (STAINING)  51 

These  last  two  methods  find  their  principal  applica- 
tion in  the  determination  of  the  composition  of  organic 
structures. 

Treatment  oJ  carbonaceous  material.  Material  con- 
taining carbon  disseminated  throughout  its  mass  can 
always  be  clarified  by  roasting  on  platinum  foil.  When 
it  is  desired  to  examine  organic  structures,  as  for  example, 
in  coal,  this  method  will  obviously  be  inapplicable.  In 
such  a  case  prolonged  treatment  with  chlorous  acid  will 
remove  the  carbonaceous  material. 


CHAPTER  IV. 

MOUNTING  OF  SANDS  AND  CRUSHED  ROCK 
MATERIAL. 

Method  of  procedure — Heavy  liquids   employed — Mounting — 

Crushed  rock  material — Schuster's  method  for  the  determination 

of  Felspars. 

In  dealing  with  loose  detrital  rocks  such  as  sands, 
loam,  etc.,  methods  of  preparation  will  be  required 
which  differ  considerably  from  those  employed  with  rock 
sections.  In  general  the  following  mode  of  procedure  will 
be  sufficient  for  the  student's  requirements.1 

1.  The  general  composition  and  condition  of  the 
material  should  be  ascertained  with  a  lens. 

2.  The  material  should  now  be  powdered,  if  neces- 
sary, and  passed  through  a  fine  sieve,  the  coarse  material 
being  rejected.     (A  30-mesh  sieve  is  a  convenient  grade, 
though  in  some  cases  a  6o-mesh  may  be  used  with  advan- 
tage.) 

3.  Removal  of  muddy  material : — This  can  be  best 
effected  by  repeated  washing  in  water ;    the  residue 


i  See  also  Hatch  and  Rastall.  Petrology  of  the  Sedimentary 
Rocks,  1913,  App.  by  T.  Crook  ;  also  Boswell,  GeoL  Mag.,  March, 
1916  ;  and  Rastall.  Camb.  Phil.  Soc.,  March,  1913. 

52 


MOUNTING  OF  SANDS  AND  CRUSHED  ROCK          53 

should  be  dried  and  examined  with  a  lens.  In  most 
cases  the  bulk  of  this  residue  will  consist  of  quartz  grains 
with  a  varying  quantity  of  felspar  and  a  small  proportion 
of  the  heavier  minerals. 

4.  The  next  operation  is  the  concentration  of  the 
heavier  minerals.  This  can  be  effected  by  panning  and 
by  the  use  of  '  heavy '  liquids.  If  a  considerable 
quantity  of  material  is  to  be  concentrated,  it  will  be 
convenient  to  remove  most  of  the  lighter  quartz  and 
felspar  by  panning.  This  is  carried  out  by  placing  a 
quantity  of  the  sample  in  a  flat-bottomed  dish.  It  is 
then  covered  with  water,  and  by  giving  the  dish  a  com- 
bined to  and  fro  and  circular  motion,  the  majority  of  the 
heavier  grains  can  be  concentrated  in  the  lower,  and 
most  of  the  quartz  and  felspar  in  the  higher  layers.  The 
latter  may  be  washed  away  and  the  process  repeated 
until  most  of  the  lighter  material  has  been  disposed  of. 
If  necessary,  this  latter  may  be  treated  again  for  re- 
covery of  the  small  proportion  of  heavy  material  which 
passed  off  with  it.  The  concentrate  should  now  be 
dried  and  examined. 

In  many  cases,  the  characters  of  the  grains  may  be 
obscured  by  cementing  material.  Treatment  with 
dilute  hydrochloric  acid  will  remove  calcareous  material, 
but  ferruginous  material  will  have  to  be  boiled  in  strong 
hydrochloric  acid.  The  concentrate  should  now  be 
dried  and  treated  with  a  'heavy*  liquid.  (In  general, 
the  most  useful  is  Bromoform  (specific  gravity  2*9). 
If  this  is  not  available,  Mercury  Potassium  Iodide 
(Thoulet's  solution)  is  the  most  suitable  of  the  many 
solutions  which  have  been  used.  It  has  the  disadvantage 
of  being  highly  corrosive,  and  is  also  more  viscous  than 


54  METHODS   IN   PRACTICAL   PETROLOGY 

Bromoform.1)  For  this  operation,  an  ordinary  glass 
funnel,  fitted  with  a  stopcock  or  a  piece  of  rubber  tubing 
and  a  pinchcock,  should  be  used. 

The  '  heavy '  liquid  is  first  poured  into  the  funnel, 
and  then  a  quantity  of  the  dried  concentrate  is  mixed 
with  it  and  well  stirred. 

Any  minerals  of  greater  specific  gravity  than  2*9  in 
the  case  of  bromoform,  or  3*3  in  that  of  Thoulet's 
solution,  will  sink  to  the  bottom,  and  can  be  drawn  off. 

The  concentrate  from  this  operation  may  be  con- 
veniently collected  in  a  second  funnel  lined  with  a  filter 
paper  and  fitted  into  the  neck  of  a  bottle  (thus  avoiding 
waste  of  the  heavy  liquid). 

The  heavy  minerals  collected  in  this  manner  may  now 
be  washed  in  a  watch  glass  or  porcelain  dish  (using 
benzene  in  the  case  of  bromoform,  and  water  in  the  case 
of  Thoulet's  solution),  and  afterwards  dried. 

A  magnet  may  be  used  to  extract  those  which  are 
magnetic,  and  further  separation  can  be  effected,  if 
desired,  by  the  use  of  liquids  of  higher  specific  gravity.2 
If  there  is  only  a  small  quantity  of  material  to  be  treated, 
the  panning  may  be  dispensed  with. 

The  heavy  minerals  concentrated  as  above  may  be 
conveniently  mounted  in  Canada  balsam.  The  '  hot 
balsam  method,'  as  used  for  rock  slices,  may  be  em- 
ployed, but  with  this  method  it  is  very  difficult  to  avoid 
air  bubbles. 


1  If  necessary  these  liquids  can  be  diluted,  the  bromoform 
with  benzene,  and  Thoulet's  solution  with  water,  giving  liquids  of 
lower  specific  gravity  for  use  with  lighter  minerals. 

2  Hatch  and  Rastall.     Petrology  of  Sedimentary  Rocks,  1913, 
p.  360. 


MOUNTING  OF  SANDS  AND  CRUSHED  ROCK        55 

The  following  mode  of  procedure  is  more  satis- 
factory in  this  respect,  but  takes  longer  time  : — Some  of 
the  material  should  be  placed  on  a  cover  glass  and 
moistened  with  turpentine.  It  should  then  be  covered 
with  a  few  drops  of  a  solution  of  balsam  in  benzene  (as 
ordinarily  sold  for  microscopic  work)  and  gently  warmed 
on  the  hot  plate  until  nearly  dry.  It  may  now  be 
covered  with  fresh  balsam  solution,  and  pressed  down 
on  to  a  glass  slide.  This  should  now  be  put  away 
to  dry.  Surplus  balsam  can  be  cleaned  off  with  benzene 
or  methylated  spirits. 

This  method  of  dealing  with  loose  sediments  may  also 
be  applied  with  advantage  to  the  examination  of  ordi- 
nary rocks.  By  this  means  it  is  often  possible  to  isolate 
accessory  minerals  which  escape  detection  in  a  rock 
slice,  owing  to  their  absence  in  the  very  small  bulk  of 
the  rock  which  such  a  slice  occupies.  It  is  also  particu- 
larly useful  for  a  rapid  determination  of  the  mineral 
composition  of  a  rock,  where  the  student  does  not  want 
to  go  to  the  trouble  of  making  a  thin  slice.  The  latter 
will,  of  course,  be  necessary  for  a  full  determination  and 
for  a  study  of  the  structure. 

The  mode  of  procedure  is  as  follows  : — 

1.  A  sample  of  the  rock  is  broken  up  in  an  iron 
mortar,  and  the  powder  obtained  is  passed  through  a  fine 
sieve ;  generally  a  6o-mesh,  though  for  fine-grained  rocks 
a  go-mesh  sieve  may  be  required  to  ensure  that  each 
grain  contains  one  mineral  only. 

2.  The  fine  dust   is  now  removed  by  washing  in 
water. 

3.  Some  of  the  residue  may  be  dried  and  mounted 


56  METHODS   IN  PRACTICAL   PETROLOGY 

without  further  treatment.  For  a  rapid  determination 
of  the  constituents,  this  will  suffice ;  but  for  a.  fuller 
examination  of  the  accessories,  the  heavy  minerals  will 
have  to  be  concentrated  with  bromuform  or  some  other 
'  heavy  '  liquid,  and  the  method  of  treatment  is  similar 
to  that  employed  with  sands. 

The  methods  employed  in  the  determination  of  the 
minerals  present  in  such  microscopical  preparations  will 
be  essentially  the  same  as  those  outlined  for  use  with 
rock  slices  (see  p.  22  et  seq.).  But  the  use  of  convergent 
light,  often  difficult  to  employ  satisfactorily  with  rock 
slices,  may  be  of  considerable  assistance  in  dealing  with 
the  minerals  present  in  fragmental  material  such  as  we 
are  considering  here. 

We  assume  that  the  microscope  which  the  student 
possesses  is  capable  of  being  used  for  this  purpose,  i.e. 
that  a  substage  condenser  and  a  Bertrand  lens  can  be 
introduced  in  addition  to  the  usual  petrological  acces- 
sories. For  a  detailed  description  of  the  use  of  converg- 
ent light  ('  Interference  figures,'  etc.)  the  student  is 
referred  to  standard  text-books  on  Mineralogy.1 

From  the  examination  in  both  parallel  and  convergent 
light,  information  as  to  the  isotropic  or  anisotropic, 
uniaxial  or  biaxial  nature  of  the  minerals  present,  should 
be  obtained,  together  with  other  physical  characters 
sufficient  for  identification  in  all  ordinary  cases. 


i  Miers.  Mineralogy,  1902,  Ch.  7  ;  also  Mennell.  Manual 
of  Petrology,  1913,  pp.  22-27  ;  also  Introduction  to  the  study  of 
Minerals,  British  Museum  (Natural  History)  guide,  1914,  PP- 
43-47  ;  and  Levy  et  Lacroix.  Les  mineraux  des  roches,  1888, 
Ch.5. 


MOUNTING  OF  SANDS  AND  CRUSHED  ROCK 


57 


In  connection  with  this  examination  of  fragmental 
material,  we  may  con- 
veniently give  here  Dr. 
Schuster's  method  for  the 
determination  of  felspars 
in  cleavage  flakes.1  It 
will  facilitate  the  ex- 
planation of  this  method 
if  we  consider  a  simple 
felspar  crystal  (fig.  6). 
In  this  we  have  perfect 
cleavages  parallel  to  P 
and  M,  and  a  less 
perfect  cleavage  parallel 
to  T.  Schuster  uses  the 
signs  +  and  -  for  sig- 
nifying the  extinction 
directions  on  the  two 
cleavages  P  and  M. 

If  the  rock,  containing  the  felspar  to  be  determined, 
is  crushed,  and  some  of  the  grains  are  mounted  as  des- 
cribed above,  it  will  usually  be  found  that  the  cleavage 
flakes  parallel  to  M  are  in  greater  abundance  than  those 
parallel  to  P. 

In  general  an  M  flake  will  be  in  the  form  of  a  parallelo- 
gram, bounded  by  the  cleavages  parallel  to  P  and  T 
respectively.  From  the  figure  it  will  be  seen  that  when 
the  cleavage  fragment  has  to  be  rotated  in  the  direction 
of  the  obtuse  angle  of  the  parallelogram  in  order  to 


FIG.  6. 


Schuster.     K  Akad  de  Wiss,  I  abth  Juli,  1879. 


METHODS   IN   PRACTICAL  PETROLOGY 


obtain  extinction,  the  sign  is  4- ,  and  when  in  the  direc- 
tion of  the  acute  angle,  the  sign  is  -. 

In  nearly  all  cases,  P  flakes  give  smaller  extinction 
angles  than  M  flakes,  and  are  determined  in  the  same 
way. 

In  the  following  table  the  Albite  and  Anorthite 
molecules  are  represented  by  Ab  and  An  respectively1: — 


FELSPAR. 

P. 

M. 

Ab 

4-     4°  30' 

+    19°  0' 

Ab13  An! 

4-     3°  38' 

4-    15°  35' 

Abg     An^ 

4-     3°  12' 

4-    13°  49' 

Ab6     Anx 

4-     2°  45' 

4-    11°  59' 

Ab5     Ani 

4-     2°  25' 

4-    10°  34' 

Ab4     Anx 

4-      1°  55' 

4-    8°  r.f 

Abo     An, 

o              1 

4-      1°  04' 

4-     4°  36' 

Abg     Anx 

-      0°  35' 

-      2°  15' 

Ab3     An2 

-      2°  12' 

-      7°  58' 

Ab,     An3 

-      2C  58' 

-    10°  26' 

Abx     Anx 

-      5°  10' 

-    16°  0' 

Abg     An6 

-      6°  50' 

-    19°  12' 

Ab3     An4 

-      7°  35' 

-    20°  52' 

Ab:     An2                      -    12°  28' 

-    26°  0' 

Ab!     An3 

-    17°  40' 

-    29°  28' 

Abx     An4 

-    21°  05' 

-    31°  10' 

Abl      An5 

-    27°  37' 

-    32°  10' 

Abi     An6 

-    27°  33' 

-    33°  29' 

Abi     An8 

-    28°  04' 

-    33°  40' 

Abi     Ani2 

-    30°  23' 

-    34°  19' 

An 

-    37°  0' 

-    36°  0' 

Microcline 

4-    15°  30' 

4-     5°  0' 

Orthoclase 

0°  | 

4-      5°  to  4-  7° 

Soda  orthoclase 

0° 

4-     9°  to  4-  12° 

Anorthoclase 

4-  1°  30'  to  4-  5°  45' 

4-  6°  to  +  9°  48' 

1  Rosenbusch. 
I.,  p.  664. 


Mikroskopische  Physiographie,    1892,   Vol. 


MOUNTING  OF  SANDS  AND  CRUSHED  ROCK 


59 


If  a  number  of  flakes  of  any  felspar  are  examined, 
two  series  of  approximately  similar  extinction  angles 
will  be  obtained,  corresponding  to  P  and  M  flakes 
respectively,  and  by  the  aid  of  the  table  the  composition 
of  the  felspar  may  be  determined  with  a  considerable 
degree  of  accuracy. 

In  the  following  table  we  give  the  six  principal 
plagioclase  felspars  with  their  composition  : — 


SILICA 

FELSPAR. 

COMPOSITION. 

PERCENTAGE. 

Albite 

Ab 

68-7% 

Oligoclase 

Ab  to  Ab3  Ani 

62-0% 

Andesine                 Abg  Ani  to  Ani  Ani 

55-6% 

Labradorite            Abi  Ani  to  Ani  Ans 

49-3% 

Bytownite 

Abi  An$  to  An 

46-6% 

Anorthite 

An 

43-2% 

APPENDIX. 
PREPARATION  OF  STAINS. 

It  may  be  convenient  to  give  methods  of  preparation 
of  stains  mentioned  in  Chapter  III.  These  are  mostly 
aniline  dyes. 

Preparation  of  Fuchsine. 

(2  grs.  Aniline. 


n      •    j    2  grs.  o-toluidine. 
Required  •{ 


•  2  grs.  p-toluidine. 
\I2  grs.  Arsenic  acid. 
Method : — Mix  the  aniline  and  two  toluidines  to- 
gether and  boil  with  the  acid  in  a  metal  bath  for  about 
an  hour.    The  resulting  product  is  soluble  in  water,  and 
is  Fuchsine  (Rosaniline  or  Magenta). 
Preparation  of  Malachite  green. 
Benzaldehyde. 
Dimethyl  aniline. 


Required 


Zinc  chloride  (solid). 


Lead  peroxide. 
v Hydrochloric  acid. 
Method : — The  benzaldehyde  and  dimethyl  aniline 
are  mixed  together  in  the  ratio  i  :  2  respectively,  and 
heated  with  excess  of  solid  zinc  chloride.    The  insoluble 
product  is  a  colourless,  crystalline  substance   (Leuco- 
base  of  Malachite  green).    This  product  is  dissolved  in 
dilute  hydrochloric  acid  to  which  a  little  peroxide  of  lead 

60 


PREPARATION  OF  STAINS  6l 

has  been  added.  It  is  well  shaken,  and  poured  into 
water,  when  the  dye  is  obtained. 

Preparation  of  Congo  Red.  This  is  a  somewhat 
complicated  process  and  involves  the  use  of  Benzidine 
dyes. 

Congo  red  is  prepared  by  the  action  of  sodium 
salicylate  on  diphenyltetrazonium  chloride.1 

Preparation  of  Aniline  Blue. 

/Magenta. 

!  Ammonia  and  Carbon  di-oxide. 
Required  -I  Aniline. 

I  Glacial  Acetic  Acid. 

\Methylated  Spirit. 

Method  : — Dissolve  some  solid  magenta  in  water  and 
pass  carbon  di-oxide  through  the  solution.  Add 
ammonia  while  the  latter  action  is  in  progress.  A 
white  precipitate  of  Rosaniline  base  will  be  obtained. 
One  gram  of  the  base  is  mixed  with  5  grs.  of  Aniline  and 
a  little  glacial  acetic  acid.  On  heating  for  a  quarter  of 
an  hour,  a  blue  precipitate  is  formed.  Extract  with 
methylated  spirit.  This  is  the  '  Aniline  Blue  '  stain. 
Preparation  of  Methylene  Blue. 

I  Nitrosodimethyl  aniline. 

„      .    ,    Ammonium  sulphide. 
Required  !„    ,      ,.     . 

Hydrochloric  acid. 

\Ferric  chloride. 

[If  Nitrosodimethyl  aniline  is  not  available,  it  should 
be  prepared  as  follows  : — A  little  dimethyl  aniline  is 
dissolved  in  dilute  hydrochloric  acid  and  a  clear  liquid 
obtained  by  violent  shaking.  When  cool,  a  few  crystals 

'  Remsen.     Organic  Chemistry,  1903,  p.  377. 


62  METHODS  IN  PRACTICAL  PETROLOGY 

of  sodium  nitrite  are  added.  Yellowish  crystals  form, 
and  some  of  the  liquid  should  then  be  hydrolised  with 
caustic  soda,  when  a  green  precipitate  is  formed,  which 
is  Nitrosodimethyl-Aniline  base.] 

Method : — The  nitrosodimethyl  aniline  is  wanned 
with  ammonium  sulphide  until  the  former  dissolves. 
When  cool,  the  solution  is  acidified  with  hydrochloric 
acid.  Ferric  chloride  is  then  added  in  excess,  and  a  blue 
colour  results  which  is  '  Methylene  Blue.' 


INDEX. 


Abrasives,  3 

Absorption,  26 

Accessory  minerals,    17,    19, 

21,  22,  26,  28,  55,  56 
Acid  rocks,  16,  17,  21,  34 
Acid,  Acetic,  49,  61 

—  Arsenic,  60 

—  Chlorous,  51 

—  Hydrochloric,  46,  47,  48, 
49,  5°,  53.  60,  61,  62 

—  Hydrofluoric,   45,   47,   48 

—  Nitric,  47,  48 
Actinolite,  23,  25 
Actinozoa,  39 
Adularia,  30 
Aegirine,  23,  24,  29,  36 
Air  bubbles,  9,  54 

—  Treatment    for    removal 
of,  9 

Albite,  23,  25,  32,  34,  58,  59 

—  twinning,  31,  33 
Alkali  Basalt,  16 

—  Dolerite,  16 

—  Gabbro,  16 

Alkaline  rocks,  16,  17,  20,  21, 

22,  35,  36 
Allivalite,  22 
Alnoite,  22 

Aluminium  chloride,  50 
Ammonia,  46,  61 
Ammonium  molybdate,   49 

—  phosphate,  49 

—  phosphomolybdate,    49 

—  sulphide,  61,  62 
Amygdales,  20 
Analcime,  23 
Analyser,  2,  24 
Anatase,  23,  25 
Andalusite,  23,  25,  28 
Andesine,  23,  25,  32,  34,  59 
Andesite,  16 


Aniline,  60,  61 

—  blue,  46 

—  Preparation  of,  61 
Aniline  dyes,  60,  61,  62 
Anorthite,  23,  25,  32,  33,  46, 

58,  59 

Anorthoclase,  58 
Apatite,  23,  25,  26 

—  Test  for,  49 
Apparatus  for  Rock  Slicing, 

2,  3 

—  Microchemical    reactions, 

44 

Aragonite,  23,  25,  39,  50 
Arenaceous  rocks,   16,  38 
Arfvedsonite,  23,  36 
Argillaceous  rocks,  16,  38 
Augite,  12,  21,  29 

—  Extinction  angle  of,  29 

—  Cleavage  angle  of,  29 

—  Colourless,  28 
Axinite,  23,  25 

Balsam,  Canada,  3,   13,   22, 

32,  33,  34,  44,  45,  48,  54.  55 
Barium  chloride,  47 
Barium  sulphate,  47 
Barkevicite,  23,  29 
Basalt,  1 6,  20 

Basic  rocks,  16,  17,  21,  34,  36 
Becke's  Line,  33 
Beeswax,  45 
Benzaldehyde,  60 
Benzene,  3,  45 
Benzidine  dyes,  61 
Bertrand  lens,  56 
Biaxial  negative  minerals,  25 

—  positive  minerals,    25 
Biotite,  20,  23,  24,  25,  26,  29 
Birefringence,  18,  24,  25,  26, 

28,  29,  30,  33-35 


64 


METHODS    IN    PRACTICAL   PETROLOGY 


Bromoform,  53 

—  Specific  gravity  of,  53,  56 
Brookite,  25 

Bytownite,  32,  59 

Calcareous  material  in  sands, 
Removal  of,  53 

—  mud,  39 

—  organic  deposits,   16,  39 
Calcic  rocks,  12,  16,  17,  35,  36 
Calcite,  12,  18,  20,  23,  25,  28, 

39,  49.  50 

Cancrinite,  23,  25,  29 

Carbonaceous     organic     de- 
posits,  1 6,  39 

—  Treatment  of,  51 
Carbonates,  Tests  for,  49 
Carbon  di-oxide,  61 

— -  di-sulphide,  44 
Carborundum,  3,  6,  8,  9,  II 

—  Wheel,  7,  8 
Carlsbad  twinning,  30 
Cassiterite,  23,  25,  26 
Caustic  soda,  62 
Cementing  material  of  Arena- 
ceous rocks,  38 

Chalybite,  23,  39 
Charnockite  series,  18 
Chert,  39 
Chiastolite,  28 
Chlorite,  23,  24,  28,  38,  46 

—  Removal  of,  48 
Chloritoid,  23,  24,  25 
Chromite,  26,  27 
Clastic  structure,  18 
Cleaning  of  microscope  slides, 

14,  45,  55 
Cleavage,  22,  24,  30,  40,  57 

—  angles,  29 

—  in  vSlates,  7,  38 

—  Secondary,  40 
Coal,  40,  51 
Cobalt  nitrate,  50 
Colour  of  minerals,  22 
Concentration       of       heavy 

minerals,  53 
Condenser,  56 
Congo  red,  46 


Congo  red,  preparation  of,  6 
Convergent  Light,  56 
Cordierite,  23,  25,  28 

—  Alteration  product  of,  28 
Corundum,  12,  23,  24,  25 
Cover  glasses,  3,  13,  44,  48 
Crushed  rock  material,  52 
Crushing,  Results  of,  20 
Crystalline  form  of  minerals, 

22,  26,  30,  34,  35,  39 
Cutting  disc,  5,  6 

Decomposition  products  of 
minerals,  26,  27,  28,  30,  38 

Determination  of  minerals,  22 

Detrital  rocks,  16,  38,  52 

Diallage,  23,  25 

Diamond  dust,  5,  6 

Dimethyl  aniline,   60,  61 

Diopside,  23,  25 

Diorite,  16,  18 

Diphenyltetrazoniurn  chlo- 
ride, 6 1 

Dolerite,  16,  19,  21 

Dolomite,  18,  23,  25,  39,  49, 

50 

Dykes,  19,  20 

Dynamic  metamorphism,  16, 
20 

Echinodermata,  39 

Eclogite,  8 

Elaeolite,  34 

Emery,  9 

Enstatite,  23,  24,  25 

Epidote,  23,  24,  25,  28,  29 

Essexite,  21 

Etching,  45 

Ether,  44,  45 

Ethyl  alcohol,  48 

Examination  of  Rock  Slices, 

15 
Extinction  angles,  24,  29,  30, 

31,  32,  58,  59 

—  Straight,  26,  29,  30 

Felspars,  i,  17,  18,  21,  22,  30, 
34.  35,  36,  48,  53 


INDEX 


Felspars,  alteration  of,  28 

—  Composition  of,  58,  59 

—  Determination  of  (Schus- 
ter), 57 

—  Staining  of,  49 
Felspathoids,  17,  21,  22,  34, 

35.  36 

Ferric  chloride,  61 
Ferromagnesian  minerals, 

17,  21,  22,  29,  36 
Ferrous  ammonium  sulphate, 

50 
Ferruginous      material      in 

sands,  Removal  of,  53 
Finishing      of      microscope 

slides,  13 

Fissile  structure,  38 
Flow  structure,  20 
Fluor,  23 
Foraminifera,  39 
Forsterite,  23 
Friable  rocks,  Treatment  of, 

Fuchsine,  46 

—  Preparation  of,  60 

Gabbrc,  16,  21 

—  Pericline  twinning  in,  31 
Garnet,  12,  23 
Gelatinisation  of  minerals,  45 
Glassy  structure,  18,  20 
Gneissic  structure,  20 
Granite,  8,  16,  20 
Granite  Porphyry,  16 
Granophyre,  16,  18 
Granular  structure,  20 
Graphite,  27 

Grinding  rock  chips,  etc.,  8, 

10,  12,  13,  45 
Groundmass  of  rocks,  19 
Gypsum,  47 

Haematite,  26,  27 
Haematoxylin,  50 
Hauyne,  23,  35,  46 

—  Test  for,  47 

•  Heavy  '  liquids,  53,  54,  56 

—  minerals,  53,  56 


Heeger's  Test,  50 
Holocrystalline  structure,  18, 

19 
Hornblende,   12,  20,  21,  23, 

24,  25,  29 

—  Extinction  angle  of,  29 

—  Cleavage  angle  of,  29 
Hybrid  rocks,  18,  41,  42 
Hypabyssal  rocks,  16,  17,  19, 

20 
Hypersthene,  23,  24,  29 

Idocrase,  23,  24,  25 
Igneous  rocks,  15,  16,  17,  18 
Ilmenite,  27 

Inclusions  in  minerals,  35 
Indices  of  thickness  of  rock 

slice,  12,  13 

Intermediate  rocks,  16, 17,  21 
Intermixing  of  rocks,  41 
Interstitial  carbonates, 

Method  of  dealing  with,  50 
Iron  ore  minerals,  21,  26 
Ironstones,  39 
Isotropic  minerals,  24 

Kaolin,  23,  25,  28 
Kyanite,  23,  24,  25 

Labradorite,  23,  25,  32,  33,  59 
Lavas,  20 
Lazurite,  46 

—  Test  for,  47 
Lead  peroxide,  60 
Lemberg's  Test,  49 
Leucite,  12,  23,  34 

Leuco  -  base     of     Malachite 

green,  60 
Leucoxene,  27 
Lime-scapolite,  46 

—  Test  for,  47 
Limestone,  12,  1 8 
Linck's  Test,  49 
Logwood  (Haematoxylin),  50 
Lubricants,  5,  6,  7 
Luxulyanite,  8 

Magenta,  60,  61 


66 


METHODS   IN    PRACTICAL    PETROLOGY 


Magnesite,  23 

Magnesium  ammonium  phos- 
phate, 49 

Magnet  with  sands.  Use  of,  54 
Magnetite,  27 
Malachite  green,  46 

—  Preparation  of,  60 
Melilite,  23,  25,  35,  46 

—  Test  for,  47 

Mercury     potassium     iodide 
(Thoulet's  solution),   53 

—  Sp.  Gr.  of,  54 
Metamorphic  rocks,    16,   17, 

20,  21,  30,  35 
Methylated  spirit,  3,  14,  45, 

55,  61 
Methylene  blue,  46 

—  Preparation  of,  61 
Mica  plate,  33 
Microchemical  methods 

(Staining),  43 

Microcline,  23,  25,  30,  31,  58 
Microlites,  32 

—  Extinction  angles  of,   32 
Microscope,  The,  2,  44,  56 
Mineralising  agents,  21 
Minerals  liable  to  give  trouble 

during  preparation  of  rock 
slice,  12 

—  of  Metamorphic  rocks,  28 

—  (Secondary),   40 
Mollusca,  39 
Monzonite,  16,  36 
Mounting  of  rock  slices,  9 
Muscovite,  20,  23,  25,  28,  29 

Nepheline,  23,  25,  34,  35,  46 

—  Decomposition  product  of, 
29 

—  Test  for,  47 
Nitrosodi methyl  aniline,  61 

—  base,  62 
Nosean,  35,  46 

—  Test  for,  47 

O-toluidine,  60 

Obsidian,  16 

Oligoclase,  23,  25,  32,  34,  59 


Olivine,  12,  21,  22,  23,  25,  29, 
30,  35,  46 

—  Test  for,  48 
Oolitic  structure,  39 
Opaque  minerals,  23,  26 
Organic    agencies    as    rock 

formers,  16,  39 
Orthoclase,  23,  25,  30  34,  58 

—  Soda,  58 

P-toluidine,  60 
Pane  bianco 's  Test,  50 
Panning,  53,  54 
'  Peg  '  structure,  35 
Pennine,  25 
Pericline  twinning,  31 
Peridotite,  8,  16,  22 
Perovskite,  23 
Perthite,  30 
Phenocrysts,    19 
Phyllites,  8,  12 
Finite,  28 

Pisolitic  structure,  39 
Pitchstone,  16 
Plagioclase  felspars,   36,   35, 
58 

—  Determination  of,  31,  57 
Plate,  Coarse  (Steel),  3,  8,  10 

—  Medium  (Glass),  3,  8,  10 

—  Fine  (Glass),  3,  9,  n 

—  Hot,  13,  55 
Platinum  foil,  51 
Pleochroism,  24,  26,  28,  29 
Plutonic  rocks,  16,  17,  18,  19, 

20 

Polariser,  2,  24 
Polishing  of  recks  and  fossils, 

9 

Porous  rocks,  Treatment  of,  7 
Porphyrite,  16 
Porphyritic  structure,  18,  19 
Potassium  ferricyanide,   50 
Preparation  of  Rock  Slices,  i 

—  for  '  staining/  44 
Putty  powder,  9 
Pyrites,  27 

Pyroclastic  rocks,  16,  40 
Pyro-Soda  developer,  47 


INDEX 


67 


Pyroxenes,  21 

—  Distinction    between 
Monoclinic  and   Rhombic, 
29 

Pyrrhotite,  27 

Quartz,  i,  12,  17,  18,  20,  21, 
23,  25,  28,  33,  34,  35,  48,  53 

—  Dolerite,  21 

—  Gabbro,  21 

—  Porphyry,  16 
-  Wedge,  33 

Radiolaria,  40 
Reagents  for  Staining,  44 
Recrystallisation,    39 
Reflected  Light,  Use  of,  23, 

26,  27 
Refractivelndices  of  Minerals, 

22,  23,  26,  28,  29,  30,  32, 

34.-  35 

—  Determination  of  relative 
(Becke),  33 

Rhyolite,  16 
Riebeckite,  23,  24,  36 
Rock  cutting  apparatus,  4,  5, 
10 

—  Slices,  Preparation  of,  i 

—  Structures,   17,   18,  36 
Rocks,   Classification  of,   37 
Rocks,  Fine  textured,  i 
Rosaniline,  60 

—  base,  6 1 

Rosenbusch,    Order   of   cry- 
stallisation, 17 

Rutile,  23,  25,  26 

Sands,  Mounting  of,  52 

Sanidine,  30 

Scapolite,  23,  25 

Schists,  8,  12 

Schuster's  method  for  deter- 
mination of  Felspars,  57 

Secondary  cleavage,  40 

Sedimentary  rocks,  15,  16, 
18,  38 

Sericitic  mica,  38 

Serpentine,  23,  25,  28,  30,  46 


Serpentine,  removal  of,  48 

Shellac,  7 

Shonkinite,  8 

Sieves  for  Sands,  etc.,  52,  55 

Silica,  39 

Silicification,  40 

Silicious  deposits,  16,  39 

Sillimanite,  23,  25,  28 

Sills,  20 

Silver  Nitrate,  47 

Silver  Sulphide,  47 

Slates,  7,  38 

Slides,  Microscope,  3 

—  Treatment  of  broken,  14 
Sodalite,  23,  35,  46 

—  Test  for,  47 
Sodium  chloride,  47 

—  nitrite,  62 

—  salicylate,  61 
Sphene,  23,  24,  25,  26 
Spinellid  minerals,  22 
Sponges,  39 

Staining          (Microchemical 

methods),  43 

Stains,  Preparation  of,  50 
Staurolite,  23,  24,  25 
Strain  shadows,  20 
Superimposed   structures,  41 
Syenite,  16,  18 

—  Porphyry,   16 

Tachylite,  12 

Thermal  metamorphism,  16 
20 

Thickness  of  slice,  Preven- 
tion of,  ii 

Thinness  of  rock  slice,  I,  n 

Thoulet's  solution,  53 

—  Sp.  Gr.  of,  54 
Titaniferous  iron  ore,  21 
Topaz,  23,  25,  26 
Tourmaline,  8,  23,  24,  25,  26 
Trachyte,  8,  16,  20 
Transference  of  slice,   13 
Tremolite,  23,  25 

Tuff,  Andesitic,  40 

—  Basaltic,  40 

—  Rhyolitic,  40 


68  METHODS   IN  PRACTICAL  PETROLOGY 


Tuff,  Trachytic,  40  Volcanic  rocks,  16,  17,  19 

Turpentine,  55  —  Vesicular,  8 
Twinning  of  Orthoclase,  30 

—  Plagioclase,  31  Wollastonite,  23,  25,  28 

—  Lamellar,  30,  33,  39 

Xenoliths,  41 

Ultrabasic  intrusions.  Minor,  —  Accidental,  41 

1 6  —  Cognate,  41 

—  lavas,  1 6 

—  rocks,  16,  17,  22  Zeolites,  20,  46 
Uniaxial   negative   minerals,  —  Removal  of,  48 

25  Zinc  chloride,  60 

—  positive  minerals,   25  Zircon,  23,  25 

Zoisite,  23,  25,  28,  29 

Variolite,  12  Zoning  of  minerals,  35 


Printed  by  W.  Hefier  &  Sons  Ltd.,  Hills  Road,  Cambridge. 


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